Methods, systems, apparatuses, and computer programs for processing tomographic images

ABSTRACT

A method, system and computer readable storage media for segmenting individual intra-oral measurements and registering said individual intraoral measurements to eliminate or reduce registration errors. An operator may use a dental camera to scan teeth and a trained deep neural network may automatically detect portions of the input images that can cause registration errors and reduce or eliminate the effect of these sources of registration errors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. Non-Provisionalpatent application Ser. No. 15/510,596 filed Mar. 10, 2017, which is aNational Stage Entry of PCT/US15/50497 filed Sep. 16, 2015 which claimspriority to U.S. Provisional Patent Appln. Nos. 62/050,881, filed Sep.16, 2014, 62/076,216, filed Nov. 6, 2014, and 62/214,830, filed Sep. 4,2015, the contents of which are incorporated by reference herein intheir entirety.

BACKGROUND Field

The present application relates generally to obtaining tomographicimages in a dental environment, and, more particularly, to methods,systems, apparatuses, and computer programs for processing tomographicimages.

Description of Related Art

X-ray radiography can be performed by positioning an x-ray source on oneside of an object (e.g., a patient or a portion thereof) and causing thex-ray source to emit x-rays through the object and toward an x-raydetector (e.g., radiographic film, an electronic digital detector, or aphotostimulable phosphor plate) located on the other side of the object.As the x-rays pass through the object from the x-ray source, theirenergies are absorbed to varying degrees depending on the composition ofthe object, and x-rays arriving at the x-ray detector form atwo-dimensional (2D) x-ray image (also known as a radiograph) based onthe cumulative absorption through the object. This process is explainedfurther in reference to FIG. 60A-FIG. 60C.

FIG. 60A shows a patient and an x-ray device 6000 positioned forobtaining an occlusal image of the mandible. A sensor or x-ray film (notshown) is disposed inside the patient's mouth. An exposure is recordedand an x-ray image is subsequently developed, as shown in FIG. 60B. Intraditional x-ray imaging, the sensor (or film) records the intensity ofthe x-rays incident thereon, which are reduced (or attenuated) by thematter which lies in their respective paths. The recorded intensityrepresents that total attenuation of the x-ray through the image volume,as illustrated in FIG. 60C.

In FIG. 60C two x-rays (ray A and ray B) are incident on two adjacentsensor elements 6008 and 6010 of an x-ray detector. Ray A travelsthrough two blocks of identical material, block 6002 and block 6004,each with an attenuation factor of 50%. Ray B travels through block 6006which also has an attenuation factor of 50%. The intensity of Ray Arecorded by sensor element 6008 is 25% of the original intensity. Theintensity of Ray B recorded by sensor element 6010 is 50% of theoriginal intensity. In a traditional x-ray image the recordedintensities are represented by light/dark regions. The lighter regionscorrespond to areas of greater x-ray attenuation and the darker regionscorrespond to areas of less, if any, x-ray attenuation. Thus, atwo-dimensional projection image produced by sensor elements 6008 and6010 will have a lighter region corresponding to sensor element 6008 anda darker region corresponding to sensor element 6010. However, from sucha two-dimensional projection image it cannot be determined that therewere two blocks of material (6002 and 6004) at different positions inthe path of Ray A, as opposed to a single block of material which causedthe same amount of x-ray attenuation. In other words, the traditionalx-ray image contains no depth information. As such, overlapping objectsmay easily obscure one another and reduce the diagnostic usefulness ofthe projection image.

Computed tomography (CT) and cone beam computed tomography (CBCT) havebeen used to acquire three-dimensional data about a patient, whichincludes depth information. The three-dimensional data can be presentedon a display screen for clinician review as a 3D rendering or as a stackof parallel 2D tomographic image slices. Each slice represents across-section of the patient's anatomy at a specified depth. While CTand CBCT machines may produce a stack of parallel 2D tomographic imageslices, these machines carry a high cost of ownership, may be too largefor use in chair-side imaging, and expose patients to a relatively highdose of x-rays.

Tomosynthesis is an emerging imaging modality that providesthree-dimensional information about a patient in the form of tomographicimage slices reconstructed from images taken of the patient with anx-ray source from multiple perspectives within a scan angle smaller thanthat of CT or CBCT (e.g., ±20°, compared with at least 180° in CBCT).Compared to CT or CBCT, tomosynthesis exposes patients to a lower x-raydosage, acquires images faster, and may be less expensive.

Typically, diagnosis using tomosynthesis is performed by assembling atomosynthesis stack of two-dimensional image slices that representcross-sectional views through the patient's anatomy. A tomosynthesisstack may contain tens of tomosynthesis image slices. Clinicians locatefeatures of interest within the patient's anatomy by evaluating imageslices one at a time, either by manually flipping through sequentialslices or by viewing the image slices as a cine loop, which aretime-consuming processes. It may also be difficult to visually graspaspects of anatomy in a proper or useful context from thetwo-dimensional images. Also, whether the tomographic images slices areacquired by CT, CBCT, or tomosynthesis, their usefulness for diagnosisand treatment is generally tied to their fidelity and quality.

Quality may be affected by image artifacts. Tomosynthesis datasetstypically have less information than full CBCT imaging datasets due tothe smaller scan angle, which may introduce distortions into the imageslices in the form of artifacts. The extent of the distortions dependson the type of object imaged. For example, intraoral tomosynthesisimaging can exhibit significant artifacts because structures within theoral cavity are generally dense and radiopaque. Still further, spatialinstability in the geometry of the tomosynthesis system and/or theobject can result in misaligned projection images which can degrade thequality and spatial resolution of the reconstructed tomosynthesis imageslices. Spatial instability may arise from intentional or unintentionalmotion of the patient, the x-ray source, the x-ray detector, or acombination thereof. It may therefore be desirable to diminish one ormore of these limitations.

SUMMARY

One or more the above limitations may be diminished by methods, systems,apparatuses, and computer programs products for processing tomographicimages as described herein.

In one embodiment, a method of identifying a tomographic image of aplurality of tomographic images is provided. Information specifying aregion of interest in at least one of a plurality of projection imagesor in at least one of a plurality of tomographic images reconstructedfrom the plurality of projection images is received. A tomographic imageof the plurality of tomographic images is identified. The identifiedtomographic image is in greater focus in an area corresponding to theregion of interest than others of the plurality of tomographic images.

In another embodiment, an apparatus for identifying a tomographic imagefrom a plurality of tomographic images. The apparatus includes aprocessor and a memory storing at least one control program. Theprocessor and memory are operable to: receive information specifying aregion of interest in at least one of a plurality of projection imagesor in at least one of a plurality of tomographic images reconstructedfrom a plurality of projection images, and identify a tomographic imageof the plurality of tomographic images. The tomographic image is ingreater focus in an area corresponding to the region of interest thanothers of the plurality of tomographic images.

In a further embodiment, a non-transitory computer-readable storagemedium storing a program which, when executed by a computer system,causes the computer system to perform a method. The method includesreceiving information specifying a region of interest in at least one ofa plurality of projection images or in at least one of a plurality oftomographic images reconstructed from the plurality of projectionimages, and identifying a tomographic image of the plurality oftomographic images. The identified tomographic image is in greater focusin an area corresponding to the region of interest than others of theplurality of tomographic images.

In still another embodiment, a method for generating clinicalinformation is provided. Information indicating at least one clinicalaspect of an object is received. Clinical information of interestrelating to the at least one clinical aspect is generated from aplurality of projection images. At least one of the steps is performedby a processor in conjunction with a memory.

In still a further embodiment, an apparatus for generating clinicalinformation. The apparatus includes a processor and a memory storing atleast one control program. The processor and the memory are operable to:receive information indicating at least one clinical aspect of anobject, and generate, from a plurality of projection images, clinicalinformation of interest relating to the at least one clinical aspect.

In yet another embodiment, a non-transitory computer readable storagemedium storing a program which, when executed by a computer system,causes the computer system to perform a method. The method includesreceiving information indicating at least one clinical aspect of anobject, and generating, from a plurality of projection images, clinicalinformation of interest relating to the at least one clinical aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings claimed and/or described herein are further described interms of exemplary embodiments. These exemplary embodiments aredescribed in detail with reference to the drawings. These embodimentsare non-limiting exemplary embodiments, in which like reference numeralsrepresent similar structures throughout the several views of thedrawings, and wherein:

FIG. 1A is a system block diagram of a tomosynthesis system according toone example embodiment herein.

FIG. 1B illustrates an example of a linear scan path used by thetomosynthesis system according to an example embodiment herein.

FIG. 1C illustrates an example of a curved scan path used by thetomosynthesis system according to an example embodiment herein.

FIG. 1D illustrates an example of a circular scan path used by thetomosynthesis system according to an example embodiment herein.

FIG. 1E illustrates an example of shadow casting from an orthogonalprojection angle.

FIG. 1F illustrates an example of shadow casting from a non-orthogonalprojection angle and the parallax induced in the image of the objects.

FIG. 2A illustrates a block diagram of an example computer system of thetomosynthesis system shown in FIG. 1A.

FIG. 2B is a flowchart illustrating a procedure for generating clinicalinformation from a tomosynthesis dataset according to an example aspectherein.

FIG. 3 is a flowchart illustrating a procedure for identifyinghigh-focus images within a tomosynthesis dataset according to an exampleembodiment herein.

FIG. 4 illustrates an example orthogonal projection image of a tooth anda region of interest indication thereon.

FIG. 5 illustrates an example focus profile within the region ofinterest of the tooth of FIG. 4.

FIG. 6 illustrates an example high-focus tomosynthesis image slice ofthe tooth of FIG. 4.

FIG. 7 illustrates another example high-focus tomosynthesis image sliceof the tooth of FIG. 4.

FIG. 8 illustrates the example orthogonal projection image of FIG. 4with a portion of FIG. 6 overlaid thereon within the region of interest.

FIG. 9 illustrates the example orthogonal projection image of FIG. 4with a portion of FIG. 7 overlaid thereon within the region of interest.

FIG. 10 illustrates an example user interface that displays high-focustomosynthesis image slices according to an example embodiment herein.

FIG. 11 illustrates an example tomosynthesis image slice with a nervecanal indicated.

FIG. 12 illustrates an example tomosynthesis image slice with a sinuscavity indicated.

FIG. 13A illustrates an example 2D radiograph.

FIG. 13B illustrates an example tomosynthesis image slice of the anatomyof FIG. 13A, wherein the nasal cavity is indicated.

FIG. 13C illustrates another example tomosynthesis image slice of theanatomy of FIG. 13A, wherein the nasal cavity is indicated.

FIG. 14A illustrates an example 2D radiograph.

FIG. 14B illustrates an example tomosynthesis image slice of the teethof FIG. 14A, wherein a crack is visible.

FIG. 14C illustrates another example tomosynthesis image slice of theteeth of FIG. 14A, wherein a crack is visible.

FIG. 15 illustrates an example user interface that displays clinicalinformation of interest according to an example embodiment herein.

FIG. 16A illustrates an example 2D radiograph.

FIG. 16B illustrates an example tomosynthesis image slice of the teethof FIG. 16A, wherein the interproximal space is visible.

FIG. 17A is a flowchart illustrating a process for generating a maskbased on a two-dimensional orthogonal projection image and using themask to guide a process for reducing image reconstruction artifacts inan intraoral tomosynthesis dataset according to an example embodimentherein.

FIG. 17B is a flowchart illustrating a subprocess of FIG. 17A, namelyfeatures of step S1708 of FIG. 17A, according to an example embodimentherein.

FIG. 17C is a flowchart illustrating a subprocess of FIG. 17A, namelyfeatures of step S1708 of FIG. 17A, according to another exampleembodiment herein.

FIG. 18 illustrates an example orthogonal projection image.

FIG. 19 illustrates a variance image corresponding to the orthogonalprojection image of FIG. 18.

FIG. 20 illustrates a gradient image corresponding to the orthogonalprojection image of FIG. 18.

FIG. 21 illustrates image statistics corresponding to the orthogonalprojection image of FIG. 18.

FIG. 22 illustrates an example binary mask image corresponding to theorthogonal projection image of FIG. 18.

FIG. 23 illustrates a tomosynthesis image slice masked by the binarymask image of FIG. 8.

FIG. 24 illustrates a graph of an example intensity transformationfunction for compressing pixel values in isolated areas of a maskedtomosynthesis image slice and a graph of an example intensitytransformation function for linear mapping of pixel values in regions ofthe masked tomosynthesis image that contain anatomic information.

FIG. 25 illustrates a graph of an example intensity transformationfunction for sinusoidal mapping of pixel values in regions of the maskedtomosynthesis image that contain anatomic information.

FIG. 26 illustrates an example reduced-artifact image slice.

FIG. 27A is a flowchart illustrating a procedure for rendering athree-dimensional (3D) image from tomosynthesis slices according to anexample embodiment herein.

FIG. 27B is a flowchart illustrating another procedure for rendering athree-dimensional (3D) image from tomosynthesis slices according to anexample embodiment herein.

FIG. 27C is a flowchart illustrating one procedure for determining athree-dimensional outline trace of an object.

FIG. 27D is a flowchart illustrating another procedure for determining athree-dimensional outline trace of an object.

FIG. 28 is a flowchart illustrating a procedure for pre-processingtomosynthesis slices according to an example embodiment herein.

FIG. 29 is a view for illustrating display of a 3D image according to anexample embodiment herein.

FIG. 30 is a view for illustrating a 3D surface rendering based on aregion-of-interest according to an example embodiment herein.

FIGS. 31A to 31C are views for illustrating measurement of distances ina 3D image according to example embodiments herein.

FIG. 32 is a flow diagram for explaining measurement of distances intomosynthesis images according to an example embodiment herein.

FIGS. 33A-D are illustrations of a two-dimensional x-ray projectionimage, a tomosynthesis slice, a distance between the tomosynthesis sliceand the x-ray detector, and the tomosynthesis slice within the imagevolume according to an example embodiment herein.

FIG. 34 is an image of a tomosynthesis slice according to an exampleembodiment herein.

FIGS. 35A-D are illustrations of a two-dimensional x-ray projectionimage, another tomosynthesis slice, a distance between the tomosynthesisslice and the x-ray detector, and the other tomosynthesis slice withinthe image volume according to an example embodiment herein.

FIG. 36 is an illustration of the other tomosynthesis slice according toan example embodiment herein.

FIGS. 37A-D are illustrations of a two-dimensional x-ray projectionimage, a tomosynthesis slice, a distance between the tomosynthesis sliceand the x-ray detector, and a vector within the image volume accordingto an example embodiment herein.

FIGS. 38A-B are illustrations of image volume with a vector therein fromtwo different orientations according to an example embodiment herein.

FIGS. 39A-C are illustrations of a two-dimensional x-ray projectionimage, a tomosynthesis slice, and a volumetric image of the image volumeaccording to an example embodiment herein.

FIGS. 40A-C are illustrations of a two-dimensional x-ray projectionimage, a tomosynthesis slice, and a volumetric image of the image volumewith a vector disposed therein according to an example embodimentherein.

FIGS. 41A-B are volumetric images of the image volume from differentorientations with a vector disposed therein,

FIG. 42A is an illustration of the human mandible.

FIG. 42B is an illustration of region R1 in FIG. 42A.

FIG. 43A is an illustration of the human maxilla from an occlusalviewpoint.

FIG. 43B is an illustration of the human mandible from an occlusalviewpoint.

FIG. 44 is a flowchart illustrating an exemplary method of measuring thethickness of the lingual and buccal plates.

FIG. 45 is a perspective view of a portion of an exemplary tomosynthesissystem 100.

FIG. 46 is an illustration of an x-ray source at different positionsduring an exemplary radiographic scan.

FIG. 47 is an illustration of an x-ray source positioned relative to apatient.

FIG. 48 is an illustration of an x-ray source positioned relative to apatient with the aid of an aiming device.

FIG. 49 is an illustration of an exemplary aiming device.

FIG. 50 is an illustration of an x-ray source at different positionsduring an exemplary radiographic scan.

FIG. 51 is an illustration of an x-ray source positioned relative to apatient.

FIG. 52 is an illustration of an x-ray source positioned relative to apatient with the aid of an aiming device.

FIG. 53A is a perspective cross-sectional view of a human mandible.

FIG. 53B is a cross-sectional view of the lingual and buccal platesshown in FIG. 53A.

FIG. 53C is an exemplary two-dimensional image from a tomosynthesisstack.

FIG. 54 is a flow diagram illustrating a procedure for tracking motionof an object according to an example embodiment herein.

FIG. 55 is a flow diagram illustrating two methods for identifyingobjects in a projection image.

FIG. 56A illustrates an example first projection image of teeth.

FIG. 56B illustrates an example second projection image of teeth.

FIG. 57A illustrates an example difference image determined from thefirst projection image of FIG. 56A and the second projection image ofFIG. 56B.

FIG. 57B illustrates an example binary image determined from thedifference image of FIG. 57A and four objects identified thereon.

FIG. 58 illustrates an example projection image of teeth and a region ofinterest indicated thereon for each identified object.

FIG. 59 illustrates a correlation algorithm for determining the amountof shift for a region of interest.

FIG. 60A is an illustration of a person positioned for an occlusal imageof the mandible according to a conventional technique.

FIG. 60B is a traditional two-dimensional x-ray image.

FIG. 60C is an illustration of two x-rays propagating through matter.

FIG. 60D is an illustration of lingual and buccal plates imagedaccording to a conventional technique.

Different ones of the Figures may have at least some reference numeralsthat are the same in order to identify the same components, although adetailed description of each such component may not be provided belowwith respect to each Figure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with example aspects described herein, methods, systems,apparatuses, and computer programs are provided for generating clinicalinformation from a tomographic dataset, and more particularly, foridentifying within an intraoral tomosynthesis dataset high-focus imagescontaining features of interest.

Tomosynthesis System

FIG. 1A illustrates a block diagram of an intraoral tomosynthesis system100 for obtaining an intraoral tomosynthesis dataset, and which isconstructed and operated in accordance with at least one exampleembodiment herein. The system 100 can be operated to obtain one or morex-ray images of an object 50 of interest, which may further include oneor more sub-object(s) 52. For example, object 50 may be a tooth (orteeth) and surrounding dentition of a patient, and sub-object(s) 52 maybe root structures within the tooth.

The system 100 includes an x-ray detector 102 and an x-ray subsystem116, both of which, including subcomponents thereof, are electricallycoupled to a computer system 106. In one example, the x-ray subsystem116 hangs from a ceiling or wall-mounted mechanical arm (not shown), soas to be freely positioned relative to an object 50. The x-ray subsystem116 further includes an x-ray source 104 mounted on a motorized stage118 and an on-board motor controller 120. The on-board motor controller120 controls the motion of the motorized stage 118.

The computer system 106 is electrically coupled to a display unit 108and an input unit 114. The display unit 108 can be an output and/orinput user interface.

The x-ray detector 102 is positioned on one side of the object 50 andthe receiving surface of the x-ray detector 102 extends in an x-y planein a Cartesian coordinate system. The x-ray detector 102 can be a smallintraoral x-ray sensor that includes, for example, a complementarymetal-oxide semiconductor (CMOS) digital detector array of pixels, acharge-coupled device (CCD) digital detector array of pixels, or thelike. In an example embodiment herein, the size of the x-ray detector102 varies according to the type of patient to whom object 50 belongs,and more particularly, the x-ray detector 102 may be one of a standardsize employed in the dental industry. Examples of the standard dentalsizes include a “Size-2” detector, which is approximately 27×37 mm insize and is typically used on adult patients, a “Size-1” detector, whichis approximately 21×31 mm in size and is typically used on patients thatare smaller than Size-2 adult patients, and a “Size-0” detector, whichis approximately 20×26 mm in size and is typically used on pediatricpatients. In a further example embodiment herein, each pixel of thex-ray detector 102 has a pixel width of 15 μm, and correspondingly, theSize-2 detector has approximately 4 million pixels in a 1700×2400 pixelarray, the Size-1 detector has approximately 2.7 million pixels in a1300×2000 pixel array, and the Size-0 detector has approximately 1.9million pixels in a 1200×1600 pixel array. The color resolution of thex-ray detector 102 may be, in one example embodiment herein, a 12-bitgrayscale resolution, although this example is not limiting, and otherexample color resolutions may include an 8-bit grayscale resolution, a14-bit grayscale resolution, and a 16-bit grayscale resolution.

The x-ray source 104 is positioned on an opposite side of the object 50from the x-ray detector 102. The x-ray source 104 emits x-rays 110 whichpass through object 50 and are detected by the x-ray detector 102. Thex-ray source 104 is oriented so as to emit x-rays 110 towards thereceiving surface of the x-ray detector 102 in at least a z-axisdirection of the Cartesian coordinate system, where the z-axis isorthogonal to the x-y plane associated with the receiving surface of thex-ray detector 102.

The x-ray source 104 can also emit x-rays 110 while positioned at eachof multiple different locations within a scan angle 112, where a 0°position in the scan angle 112 corresponds to the position for emittingx-rays 110 along the z-axis. In one example embodiment herein, the userinitially positions the x-ray subsystem 116, and hence, also the x-raysource 104, to a predetermined starting position relative to the object50. The computer system 106 then controls the on-board motor controller120 to move the x-ray source 104 via the motorized stage 118, based onthe known starting position, to step through each of the differentlocations within the scan angle 112. The computer system 106 controlsthe x-ray source 104 to cause the source 104 to emit x-rays 110 at eachof those locations.

The centroid of the x-rays 110 passes through a focal spot 122 at eachof the different locations within the scan angle 112. The focal spot 122may be, for example, located close to the detector such that x-rays 110emitted from the x-ray source 104 positioned at the outer limits of thescan angle 112 are aimed at and do not miss the x-ray detector 102. InFIG. 1A, the 0° position is represented in x-ray source 104, whilereference numerals 104 a and 104 b represent the same x-ray source 104but in two other example positions within the scan angle 112. The scanangle 112 can be, for example, ±20° from the 0° position, although thisexample is not limiting.

Additionally, the motion of x-ray source 104 along the scan angle 112may form different scan paths, such as, for example, a linear scan 130shown in FIG. 1B, a curved scan 132 shown in FIG. 1C, or a circular scan134 shown in FIG. 1D. In the linear scan 130 (FIG. 1B), the x-ray source104 moves linearly in an x-y plane while emitting x-rays 110 toward thefocal spot 122, forming a triangular sweep. In the curved scan 132 (FIG.1C), the x-ray source 104 moves in an arc while emitting x-rays 110toward the focal spot 122, forming a fan beam sweep. In the circularscan 134 (FIG. 1D), the x-ray source 104 rotates around the z-axis whileemitting x-rays 110 toward the focal spot 122, forming a conical beamsweep. The scan positions also may be arranged in any particular one ormore planes of the Cartesian coordinate system.

As emitted x-rays 110 pass through the object 50, photons of x-rays 110will be more highly attenuated by high density structures of the object50, such as calcium-rich teeth and bone, and less attenuated by softtissues, such as gum and cheek. One or more of the attenuatingstructures can be sub-object(s) 52. X-rays 110 passing through andattenuated by object 50, are projected onto x-ray detector 102, whichconverts the x-rays 110 into electrical signals and provides theelectrical signals to computer system 106. In one example embodiment,the x-ray detector 102 may be an indirect type of detector (e.g., ascintillator x-ray detector) that first converts x-rays 110 into anoptical image and then converts the optical image into the electricalsignals, and in another example embodiment, the x-ray detector 102 maybe a direct type of detector (e.g., a semiconductor x-ray detector) thatconverts x-rays 110 directly into the electrical signals. The computersystem 106 processes the electrical signals to form a two-dimensionalprojection image of the object 50. In one example embodiment herein, theimage size of the two-dimensional projection image corresponds to thedimensions and the number of pixels of the x-ray detector 102.

The system 100 can collect a plurality of projection images, asdescribed above, by first positioning the x-ray source 104 at differentangles, including at least the 0° position, and emitting x-rays 110 ateach of those different angles through object 50 towards x-ray detector102. For example, the plurality of projection images may include a totalof fifty-one projections: one orthogonal projection image, obtained whenthe x-ray source is at the 0° position, and fifty projection images,each obtained when the x-ray source 104 is positioned at differentangles within a range of ±20° from the z-axis (corresponding to the scanangle 112). In other example embodiments, the number of projectionimages may range from twenty-five to seventy. Because the orthogonalprojection image is obtained when the x-ray source is at the 0°position, the orthogonal projection image has the same appearance as aconventional x-ray image. That is, the two-dimensional orthogonalprojection image has no depth perception, and one or more sub-object(s)52 within object 50 may appear overlaid one on top of another in theorthogonal projection image, as represented in FIG. 1E, for example. Onthe other hand, sub-object(s) 52 at different depths of the z-axiswithin object 50 undergo varying degrees of parallax when imaged fromdifferent angles along the scan angle 112, as represented in FIG. 1F,for example.

The computer system 106 processes the plurality of projection images toreconstruct a series of two-dimensional tomosynthesis image slices, alsoknown as a tomosynthesis stack of images, in a manner to be describedbelow. Each image slice is parallel to the plane in which the receivingsurface of the x-ray detector 102 extends and at different depths of thez-axis.

The computer system 106 further processes the tomosynthesis image slicesin a manner to be described below, to generate clinically relevantinformation related to object 50 (e.g., a patient's dental anatomy), andin a further example embodiment herein, related to sub-object(s) 52. Theextracted information may include the identification, within thetomosynthesis stack of images, of high-focus images that containfeatures of interest therein. In one example embodiment herein, thecomputer system 106 obtains input from a user via input unit 114 and/ordisplay unit 108 to guide the further processing of the tomosynthesisslices.

The orthogonal projection image, one or more image slices of thetomosynthesis stack, and the extracted information are provided by thecomputer system 106 for display to the user on the display unit 108.

Compared to a dental CBCT system, the intraoral tomosynthesis imagingsystem 100 carries a lower cost of ownership, can acquire images fasterand with higher resolution (e.g., a per pixel resolution ofapproximately 20 μm, compared to a per pixel resolution of 100-500 μmwith CBCT), and exposes patients to a lower x-ray dose (e.g.approximately an order of magnitude lower in some cases, owing in partto a smaller field of view, a smaller scan angle, and the need to onlypenetrate the anatomy between the x-ray source 104 and the x-raydetector 102, rather than the complete jaw). Additionally, in someexample embodiments herein, the intraoral tomosynthesis system 100 canresemble a conventional x-ray radiography system, and can use the sameor substantially similar equipment, such as, for example, the ceiling-or wall-mounted mechanical arm for positioning the x-ray source 104, asimilarly-sized x-ray source 104, and the intraoral x-ray detector 102.Accordingly, operation of the intraoral tomosynthesis system 100 is morefamiliar and less complex to a clinician, compared to dental CBCT, andalso can be used chair-side.

Computer System for Tomosynthesis Imaging

Having described a system 100 for acquiring a tomosynthesis dataset andfor generating clinically relevant information from a tomosynthesisdataset, including the identification of high-focus images containingfeatures of interest, reference will now be made to FIG. 2A, which showsa block diagram of a computer system 200 that may be employed inaccordance with at least some of the example embodiments herein.Although various embodiments are described herein in terms of thisexemplary computer system 200, after reading this description, it willbecome apparent to a person skilled in the relevant art(s) how toimplement the invention using other computer systems and/orarchitectures.

FIG. 2A illustrates a block diagram of the computer system 200. In oneexample embodiment herein, at least some components of the computersystem 200 (such as all those components, or all besides component 228)can form or be included in the computer system 106 shown in FIG. 1A. Thecomputer system 200 includes at least one computer processor 222 (alsoreferred to as a “controller”). The computer processor 222 may include,for example, a central processing unit, a multiple processing unit, anapplication-specific integrated circuit (“ASIC”), a field programmablegate array (“FPGA”), or the like. The processor 222 is connected to acommunication infrastructure 224 (e.g., a communications bus, across-over bar device, or a network).

The computer system 200 also includes a display interface (or otheroutput interface) 226 that forwards video graphics, text, and other datafrom the communication infrastructure 224 (or from a frame buffer (notshown)) for display on a display unit 228 (which, in one exampleembodiment, can form or be included in the display unit 108). Forexample, the display interface 226 can include a video card with agraphics processing unit.

The computer system 200 also includes an input unit 230 that can be usedby a user of the computer system 200 to send information to the computerprocessor 222. In one example embodiment herein, the input unit 230 canform or be included in the input unit 114. For example, the input unit230 can include a keyboard device and/or a mouse device or other inputdevice. In one example, the display unit 228, the input unit 230, andthe computer processor 222 can collectively form a user interface.

In an example embodiment that includes a touch screen, for example, theinput unit 230 and the display unit 228 can be combined, or represent asame user interface. In such an embodiment, a user touching the displayunit 228 can cause corresponding signals to be sent from the displayunit 228 to the display interface 226, which can forward those signalsto a processor such as processor 222, for example.

In addition, the computer system 200 includes a main memory 232, whichpreferably is a random access memory (“RAM”), and also may include asecondary memory 234. The secondary memory 234 can include, for example,a hard disk drive 236 and/or a removable-storage drive 238 (e.g., afloppy disk drive, a magnetic tape drive, an optical disk drive, a flashmemory drive, and the like). The removable-storage drive 238 reads fromand/or writes to a removable storage unit 240 in a well-known manner.The removable storage unit 240 may be, for example, a floppy disk, amagnetic tape, an optical disk, a flash memory device, and the like,which is written to and read from by the removable-storage drive 238.The removable storage unit 240 can include a non-transitorycomputer-readable storage medium storing computer-executable softwareinstructions and/or data.

In alternative embodiments, the secondary memory 234 can include othercomputer-readable media storing computer-executable programs or otherinstructions to be loaded into the computer system 200. Such devices caninclude a removable storage unit 244 and an interface 242 (e.g., aprogram cartridge and a cartridge interface similar to those used withvideo game systems); a removable memory chip (e.g., an erasableprogrammable read-only memory (“EPROM”) or a programmable read-onlymemory (“PROM”)) and an associated memory socket; and other removablestorage units 244 and interfaces 242 that allow software and data to betransferred from the removable storage unit 244 to other parts of thecomputer system 200.

The computer system 200 also can include a communications interface 246that enables software and data to be transferred between the computersystem 200 and external devices. Examples of the communicationsinterface 246 include a modem, a network interface (e.g., an Ethernetcard or an IEEE 802.11 wireless LAN interface), a communications port(e.g., a Universal Serial Bus (“USB”) port or a FireWire® port), aPersonal Computer Memory Card International Association (“PCMCIA”)interface, and the like. Software and data transferred via thecommunications interface 246 can be in the form of signals, which can beelectronic, electromagnetic, optical or another type of signal that iscapable of being transmitted and/or received by the communicationsinterface 246. Signals are provided to the communications interface 246via a communications path 248 (e.g., a channel). The communications path248 carries signals and can be implemented using wire or cable, fiberoptics, a telephone line, a cellular link, a radio-frequency (“RF”)link, or the like. The communications interface 246 may be used totransfer software or data or other information between the computersystem 200 and a remote server or cloud-based storage (not shown).

One or more computer programs (also referred to as computer controllogic) are stored in the main memory 232 and/or the secondary memory234. The computer programs also can be received via the communicationsinterface 246. The computer programs include computer-executableinstructions which, when executed by the computer processor 222, causethe computer system 200 to perform the procedures as described herein(and shown in figures), for example. Accordingly, the computer programscan control the computer system 106 and other components (e.g., thex-ray detector 102 and the x-ray source 104) of the intraoraltomosynthesis system 100.

In one example embodiment herein, the software can be stored in anon-transitory computer-readable storage medium and loaded into the mainmemory 232 and/or the secondary memory 234 of the computer system 200using the removable-storage drive 238, the hard disk drive 236, and/orthe communications interface 246. Control logic (software), whenexecuted by the processor 222, causes the computer system 200, and moregenerally the intraoral tomosynthesis system 100, to perform theprocedures described herein.

In another example embodiment hardware components such as ASICs, FPGAs,and the like, can be used to carry out the functionality describedherein. Implementation of such a hardware arrangement so as to performthe functions described herein will be apparent to persons skilled inthe relevant art(s) in view of this description.

Having provided a general description of the tomosynthesis system 100,techniques for processing data from the tomosynthesis system 100 (or asthe case may be a CT or CBCT machine as well) will be described below.As one of ordinary skill will appreciate, description corresponding toone technique may be applicable to another technique described herein.

Generating Clinical Information from a Dataset

Generally, for x-ray images to have value and utility in clinicaldiagnosis and treatment, they should have high image fidelity andquality (as measured by resolution, brightness, contrast,signal-to-noise ratio, and the like, although these example metrics arenot limiting) so that anatomies of interest can be clearly identified,analyzed (e.g., analysis of shape, composition, disease progression,etc.), and distinguished from other surrounding anatomies.

In addition to providing tomosynthesis image slices with good imagefidelity and quality (although such is not necessary), an intraoraltomosynthesis system 100 according to example aspects herein augmentsthe tomosynthesis image slices by automatically or semi-automaticallygenerating clinical information of interest about the imaged object 50and presenting the same to the clinician user. In an example embodimentherein, the clinical information of interest relates to anatomicalfeatures (such as sub-object(s) 52) located at a depth within the object50, and such anatomical features may not be readily apparent in thetomosynthesis image slices under visual inspection by the clinician userand also may not be visible in a conventional 2D radiograph due tooverlapping features from other depths.

The intraoral tomosynthesis system 100 will now be further described inconjunction with FIG. 2B, which shows a flow diagram of a process forgenerating clinical information of interest according to an exampleembodiment herein.

The process of FIG. 2B starts at Step S201, and in Step S202, thetomosynthesis system 100 acquires a plurality of projection images ofthe object 50 over a scan angle 112.

In Step S204, the computer system 106 processes the plurality ofprojection images to reconstruct a series of two-dimensionaltomosynthesis image slices (also known as a tomosynthesis stack), eachimage slice representing a cross-section of the object 50 that isparallel to the x-ray detector 102 and each slice image also beingpositioned at a different, respective, location along the z-axis (i.e.,in a depth of the object 50) than other image slices. (Thereconstruction of the tomosynthesis stack in Step S204 can besubstantially the same process as that of Step S304 of FIG. 3 describedin greater detail herein below.)

In Step S206, the computer system 106 receives, via input unit 114and/or display unit 108, a guidance from a clinician user indicating aclinical aspect of interest. In an example embodiment herein, thereceived guidance may be a user selection from among a predeterminedlist of tools presented by the computer system 106.

The guidance received in Step S206 may be, for example, and withoutlimitation, a selection of at least one region of interest on at leastone of the projection images or the tomosynthesis image slices, at leastone anatomy of interest (e.g., mental foramen, nerve canal, sinus floor,sinus cavity, nasal cavity, periodontal ligament, lamina dura, or otherdental anatomies), a type of dental procedure (e.g., an endodonticprocedure, a periodontic procedure, an implantation, caries detection,crack detection, and the like), a measurement inquiry (e.g., a distancemeasurement, a volumetric measurement, a density measurement, and thelike), or any combination thereof

In Step S208, the computer system 106 processes the tomosynthesis stackto generate information that is relevant to the clinical aspect ofinterest indicated by the guidance received in Step S206. In an exampleembodiment herein, the computer system 106 performs a processing in StepS208 that is predetermined to correspond to the received guidance.

Non-limiting examples of tomosynthesis stack processing that can beperformed in Step S208 (and the information generated thereby) for aparticular received guidance are as follows.

In an example embodiment herein where the received guidance is aselection of at least one region of interest on at least one of theprojection images or the tomosynthesis image slices, the computer system106 processes the tomosynthesis stack according to a process describedfurther herein below with reference to FIG. 3.

Where the received guidance is at least one anatomy of interest (e.g.,mental foramen, nerve canal, sinus floor, sinus cavity, nasal cavity,periodontal ligament, lamina dura, or other dental anatomies), thecomputer system 106 processes the tomosynthesis stack to identify theanatomy of interest (e.g., by way of image segmentation). One or moreimage segmentation techniques may be used to identify the anatomy ofinterest including, for example, a Hough transformation, a gradientsegmentation technique, and a minimal path (geodesic) technique, whichare discussed in further detail below. The computer system 106generates, as generated information, a display image that indicates theanatomy of interest (e.g., by highlighting, outlining, or the like). Inone example embodiment herein, the display can be the tomosynthesisimage slices with the identified anatomy indicated thereon or a 3Drendering of the identified anatomy. For example, FIG. 11 illustrates atomosynthesis image slice with a nerve canal 1102 outlined thereon, andFIG. 12 illustrates a tomosynthesis image slice with a sinus cavity 1202outlined thereon. As another example, FIG. 13A illustrates a 2Dradiograph of a patient's anatomy, wherein a nasal cavity is lessclearly defined, but, by virtue of performing Step S208 on atomosynthesis dataset acquired from the same anatomy, the computersystem 106 identifies the nasal cavity and indicates at least the nasalcavity walls 1302, 1304, 1306, and 1308 on the tomosynthesis imageslices shown in FIGS. 13B and 13C.

If the received guidance is a type of dental procedure (e.g., anendodontic procedure, a periodontic procedure, an implantation, cariesdetection, crack detection, and the like), the computer system 106generates information specific to the dental procedure.

For example, for a guidance indicating an endodontic root canalprocedure, the computer system 106 processes the tomosynthesis datasetto identify root canals and generates a display of the identified rootcanals as the generated information (as discussed below). For example,the generated information can be the tomosynthesis image slices with theroot canals highlighted and/or a 3D rendering of the root canals. In anadditional example embodiment herein, the computer system 106 cangenerate spatial information related to the shape of the root canal,such as, for example, its location, curvature, and length.

For a received guidance indicating an implantation, the computer system106, in an example embodiment herein, processes the tomosynthesis stackand generates, as the generated information, locations of anatomicallandmarks of interest for an implant procedure, such as, for example, alocation of the nerve canal, a location of the sinus floor, a locationof the gingival margin, and a location of the buccal plate, throughimage segmentation. The computer system 106 can also generate, as thegenerated information, a 3D rendering of the jaw with the teethvirtually extracted.

For a received guidance indicating caries detection, the computer system106, in an example embodiment herein, processes the tomosynthesis stackto detect caries and generates, as the generated information, thelocations of carious lesion(s). In one embodiment, the guidance mayinclude information that the computer system 106 uses to evaluatesegmented regions and identify one or more of the regions as cariousregions. Such information may include, for example, expected region sizeand attenuation amounts for a carious region. The locations of cariouslesion(s) can be in the form of the tomosynthesis image slices with thecarious region(s) highlighted thereon or a 3D rendering of the affectedtooth of teeth with the carious volume(s) highlighted thereon.

For a received guidance indicating crack detection, the computer system106, in an example embodiment herein, processes the tomosynthesis stackto detect cracks and generates, as the generated information, thelocation of any cracks in the imaged tooth or teeth. In some exampleembodiments herein, the location of a crack can be in the form of thetomosynthesis image slices with the crack indicated thereon or a 3Drendering of the affected tooth of teeth with the crack indicatedthereon. For example, the computer system 106 can process atomosynthesis dataset to identify cracks in the imaged teeth (usingimage segmentation), and then generate the tomosynthesis image slicesshown in FIGS. 14B and 14C with the identified cracks 1402 and 1404indicated thereon, respectively.

In an example embodiment herein where the received guidance is ameasurement inquiry (e.g., a distance measurement, an 2D area or 3Dvolumetric measurement, a density measurement, and the like), thecomputer system 106 processes the tomosynthesis stack to calculate therequested measurement as the generated information. For example, thecomputer system 100 can calculate, as the generated information, adistance between at least two user-selected points in the tomosynthesisdataset, a distance between two or more anatomies identified in themanner described above, an area or volume of an identified anatomy or ofa user-selected region of the tomosynthesis dataset, or a density of anidentified anatomy or of a region of the tomosynthesis dataset.

In Step S210, the computer system 106 presents the information generatedin Step S208 to the user on the display unit 108. In an exampleembodiment herein, the computer system 106 can present the informationgenerated in Step S208 by way of a user interface displayed on displayunit 108.

FIG. 15 illustrates a particular example of a user interface forpresenting, in accordance with Step S210, information generated in StepS208 in response to a guidance received in Step S206 to locate themental foramen 1502. In the particular example shown in FIG. 15, thecomputer system 106 displays a tomosynthesis image slice 1504 with thelocation of the mental foramen 1502 indicated thereon, a 3D rendering ofa tooth 1506 with the location of the mental foramen 1502 indicated in3D space in relation to the 3D-rendered tooth 1506, and a distancemeasurement 1508 from the apex of the 3D-rendered tooth 1506 to themental foramen 1502.

The process of FIG. 2B ends at Step S212.

As can be appreciated in view of the foregoing, by virtue of theprocessing being performed on a tomosynthesis stack, which includes 3Dinformation about the object 50 as explained above, the generatedinformation also provides to the clinician user a depth information anddepth context about the object 50 that may not be readily apparent inthe tomosynthesis image slices under visual inspection by the clinicianuser and also may not be visible in a conventional 2D radiograph due tooverlapping features from other depths.

As one particular example of useful depth information provided to auser, the tomosynthesis system 100 performing the process of FIG. 2B canautomatically detect interproximal caries between teeth, because aninterproximal space (e.g., space 1602 on FIG. 16B) between teeth isvisible in at least one of the tomosynthesis image slices but would beobscured by overlapping anatomies in a conventional 2D radiograph of thesame region (e.g., FIG. 16A). As another particular example of a depthinformation, the tomosynthesis system 100 performing the process of FIG.2B can automatically detect dental cracks (e.g., cracks 1402 and 1404 onFIGS. 14B and 14C, respectively) in individual ones of the tomosynthesisimage slices, which also may be obscured by overlapping anatomies in aconventional 2D radiograph (e.g., FIG. 14A).

Additionally, by virtue of using the computer system 106 to perform atleast part of the process shown in FIG. 2B and described above, thetomosynthesis system 100 can be controlled to acquire images of lowerfidelity and lower quality, thus potentially lowering the x-ray exposureto the patient and reducing image acquisition time, even whilegenerating and presenting clinical information of high value andutility.

Identifying High-Focus Images within a Dataset

The intraoral tomosynthesis system 100 will now be further described inconjunction with FIG. 3, which shows a flow diagram of a processaccording to an example embodiment herein for identifying high-focusimages within a tomosynthesis dataset. Prior to starting the process,the x-ray detector 102 and x-ray source 104 are aligned manually by auser to a starting position, as described above, in one exampleembodiment herein.

The process of FIG. 3 starts at Step S301, and, in Step S302, theintraoral tomosynthesis system 100 acquires a plurality of projectionimages of object 50 over a scan angle 112 (which may be predetermined),including the orthogonal projection image, in the manner describedabove. For example, the x-ray source 104 is moved by the motorized stage118 and control circuitry 120 to different positions within the scanangle 112, and the computer system 106 controls the x-ray source 104 toemit x-rays 110 at each position. In one example embodiment herein,x-ray source 104 is scanned, by pivoting at a point along the z-axis,from −20° from the z-axis to +20° from the z-axis in evenly distributedincrements of 0.8° to provide 51 scan angles, including the 0° position,although this example is not limiting. The x-rays 110 then pass throughand are attenuated by the object 50 before being projected onto thex-ray detector 102. The x-ray detector 102 converts the x-rays 110 intoelectrical signals (either directly or indirectly, as described above)and provides the electrical signals to the computer system 106. Thecomputer system 106 processes the electrical signals collected at eachscan angle position to acquire the plurality of projection images, eachimage comprising an array of pixels. The image acquired with the x-raysource 104 at the 0° position is also referred to herein as anorthogonal projection image.

In one example embodiment herein, the color depth of each pixel value ofthe projection images may be 12-bit grayscale, and the dimensions of theprojection images correspond to the standard dental size of the x-raydetector 102, as described above. For example, a Size-2 detector mayproduce projection images that are approximately 1700×2400 pixels insize, a Size-1 detector may produce projection images that areapproximately 1300×2000 pixels in size, and a Size-0 detector mayproduce projection images that are approximately 1200×1600 pixels insize.

In Step S304, the computer system 106 processes the plurality ofprojection images acquired in Step S302 using a reconstruction techniquein order to reconstruct a series of two-dimensional tomosynthesis imageslices and may also perform deblurring and other image enhancements, aswill be described further herein. Each reconstructed image slice is atomographic section of object 50 comprising an array of pixels, that is,each image slice represents a cross-section of object 50 that isparallel to the x-y plane in which the receiving surface of the x-raydetector 102 extends, has a slice thickness along the z-axis, and ispositioned at a different, respective location along the z-axis thanother image slices. The slice thickness is a function of thereconstruction technique and aspects of the geometry of the system 100,including, primarily, the scan angle 112. For example, each image slicemay have a slice thickness of 0.5 mm by virtue of the geometry of thesystem 100 and the reconstruction technique. The desired location ofeach reconstructed image slice along the z-axis is provided as an inputto the reconstruction performed in Step S304 either as a pre-programmedparameter in computer system 106 or by user input via input unit 114and/or display unit 108. By example only, the computer system 106 can beinstructed to reconstruct, from the plurality of projection images, afirst image slice that is one millimeter (1 mm) away from the surface ofx-ray detector 102 along the z-axis, a last image slice being at fifteenmillimeters (15 mm) away from the surface of the x-ray detector 102, andimage slices between the first image slice and the last image slice atregular increments along the z-axis of two-hundred micrometers (200 μm),for a total of seventy-one image slices.

Reconstruction of the tomosynthesis image slices in Step S304 may beperformed in accordance with any existing or later developedreconstruction technique. For example, a shift-and-add method, filteredbackprojection, matrix inversion tomosynthesis, generalized filteredbackprojection, SIRT (simultaneous iterative reconstruction technique),or algebraic technique, among others, may be used. In one exampleembodiment herein, reconstruction of the tomosynthesis image slices inStep S304 utilizes a shift-and-add technique. The shift-and-addtechnique utilizes information about the depth of sub-object(s) 52 alongthe z-axis that is reflected in the parallax captured by the pluralityof projection images, as described above. According to this exampleembodiment, an image slice is reconstructed by first spatially shiftingeach projection image by an amount that is geometrically related to thedistance between the image slice and the focal spot 122 along thez-axis. The shifted projection images are then averaged together toresult in the image slice, where all sub-objects 52 in the plane of theimage slice are in focus and sub-objects 52 outside of that plane areout of focus and blurry. This shift-and-add process is repeated for eachimage slice to be reconstructed. In the case of the image slicecorresponding to the x-y plane that includes the focal spot 122, theprojection images are averaged together without first shifting becausesub-objects 52 are already in focus for that plane.

The foregoing describes a basic shift-and-add reconstruction technique.In one example embodiment herein, a deblurring technique thatsubstantially reduces or removes blurry, out-of-plane sub-objects froman image slice can be performed in conjunction with the reconstructiontechnique (whether shift-and-add or another technique). Examples ofdeblurring techniques that can be employed include, for example, spatialfrequency filtering, ectomography, filtered backprojection, selectiveplane removal, iterative restoration, and matrix inversiontomosynthesis, each of which may be used in Step S304 to deblur imagesreconstructed by the shift-and-add reconstruction technique (or anotherreconstruction technique, if employed).

In another example embodiment herein, Step S304 also can include thecomputer system 106 performing further automated image enhancements suchas, for example, image sharpening, brightness optimization, and/orcontrast optimization, on each reconstructed (and deblurred, wheredeblurring is performed) image slice in a known manner.

Additionally, in another example embodiment herein, the dimensions,position, and orientation of each image slice reconstructed in Step S304are the same as the corresponding characteristics of the orthogonalprojection image. Thus, when tomosynthesis image slices (or portionsthereof) and the orthogonal projection image are overlaid over oneanother, corresponding anatomical features appearing in the images willbe overlapped and aligned without scaling, rotation, or othertransformation of the images.

In Step S306, the computer system 106 assembles the tomosynthesis imageslices into an ordered stack of two-dimensional tomosynthesis imagesslices. Each image slice is assembled into the stack according to itscorresponding location in object 50 along the z-axis, such that theimage slices in the stack are ordered along the z-axis in the order ofsuch locations along that axis. Each image slice is associated with animage number representing the position of that image in the orderedstack. For example, in a stack of sixty tomosynthesis image slicesassembled from sixty tomosynthesis image slices, image number one can bethe image slice closest to the x-ray detector 102 and image number sixtycan be the image slice farthest from the x-ray detector 102. In oneexample embodiment herein, images of the plurality of projection imagesand image slices of the tomosynthesis stack have the same dimensionalresolution and color depth characteristics.

After Step S306, control passes to Step S310, which will be describedbelow. Before describing that step, Step S308 will first be described.Like Step S304, Step S308 is performed after Step S302 is performed.

In Step S308, the orthogonal projection image is extracted from theplurality of projection images acquired in Step S302. Because, asdescribed above, the orthogonal projection image is defined as theprojection image captured while the x-ray source 104 is in the 0° scanangle position, no reconstruction is necessary to extract that image. Inone example embodiment herein, the orthogonal projection image isextracted and stored in the main memory 232, although it may be storedinstead in the secondary memory 234, and can be retrieved therefrom fordisplay in Step S310 and/or Step S322. In another example embodimentherein, the extracted orthogonal projection image may undergo automatedimage enhancements (performed by computer system 106) such as, forexample, image sharpening, brightness optimization, and/or contrastoptimization, in a known manner.

In Step S310, the stack of tomosynthesis image slices assembled in StepS306 and the orthogonal projection image extracted in Step S308 aredisplayed on the display unit 108. In one example embodiment herein, thedisplaying can be performed as to show the entire stack, or one or moreselected image slices of the stack, using display unit 108, andinteractive controls (e.g. via display unit 108 and/or input device 114)enable a user to select between those two options, and to select one ormore image slices for display, and also to select one or more particularregions of interest in the image(s) for display (whether in zoom ornon-zoom, or reduced fashion). In a further example embodiment, asdescribed below, stack controls 1016 illustrated in FIG. 10 are providedand can include a scroll bar, which enables the user to manually selectwhich image slice is displayed on the display unit 108, and/or caninclude selectable control items, such as play, pause, skip forward, andskip backward, (not shown) to enable the user to control automaticdisplay of the tomosynthesis stack, as a cine loop for example, on thedisplay unit 108.

In Step S312, the computer system 106 receives, via input unit 114and/or display unit 108, an indication of a region of interest from auser. In one example embodiment herein, the user indicates a region ofinterest on the orthogonal projection image displayed on the displayunit 108 in Step S310. In an alternative example embodiment herein, theuser indicates a region of interest on a tomosynthesis image slicedisplayed on the display unit 108 in Step S310.

Additionally, the region of interest may be a rectangular marquee (orany other outlining tool, including but not limited to a hand-drawnoutline, a marquee of a predetermined shape, and the like) drawn on theorthogonal projection image or tomosynthesis image slice displayed onthe display unit 108 in Step S310. For example, FIG. 4 illustrates anexample of a rectangular region of interest 402 drawn over an exampleorthogonal projection image, although this example is not limiting.

In Step S314, the computer system 106 applies a focus function todetermine the degree to which the region of interest of each image slicein the tomosynthesis stack is in focus and assigns a focus factor toeach image slice based on the results of the focus function. In oneexample embodiment herein, prior to applying the focus function, thecomputer system 106 pre-processes image slices in the tomosynthesisstack to reduce image artifacts, such as ringing, motion blur,hot-pixels, and x-ray generated noise. In a further example embodimentherein, the image pre-processing includes applying a Gaussian blurfilter to each image slice in a known manner.

After pre-processing image slices, if performed, the computer system 106applies the focus function to each image slice in the tomosynthesisstack. For example, first, the computer system 106 extracts a region ofinterest image, which is a portion of the tomosynthesis slice imagecorresponding to the region of interest received in Step S312. Then, theregion of interest image is padded on all sides to avoid orsubstantially minimize possible creation (if any) of image processingartifacts in a border region during subsequent processing in Step S312,including the deriving of a variance image as described below. The pixelvalues of the padding may be, for example, a constant value (e.g.,zero), an extension of the border pixels of the region of interestimage, or a mirror image of the border pixels of the region of interestimage. After the region of interest image has been padded, a varianceimage is derived by iterating a variance kernel operator, for example, a5×5 pixel matrix, through each pixel coordinate of the region ofinterest image. At each iterative pixel coordinate, the statisticalvariance of pixel values of the region of interest image within thevariance kernel operator is calculated, and the result is assigned to acorresponding pixel coordinate in the variance image. Then, the varianceimage is cropped to the same size as that of the unpadded region ofinterest image. Lastly, the focus factor is calculated as thestatistical mean of the pixel values in the cropped variance image.Accordingly, a high focus factor corresponds to a high mean variancewithin the region of interest image. The focus factor is assigned to theimage slice, and correspondingly, the focus factor is associated withthe image number of the image slice to which it is assigned. Theforegoing process is applied to each slice, for example, by serialiteration and/or in parallel, to assign a focus factor to each imageslice.

In the preceding example embodiment, performing the focus function onthe region of interest portion of each image slice instead of on thefull view of each image slice facilitates the process of FIG. 3 inidentifying the z-axis location of images slices that are in high focuswithin the region of interest relative to other images slices. The focusfunction can be performed on the full view of image slices; however,focusing techniques performed on the full view may, in at least somecases, function less effectively to identify the location of high-focusimages, because the full view of most, if not all, image slices containsboth in-focus and out-of-focus regions.

In Step S316, the computer system 106 creates a focus profile from aseries of the focus factors assigned in Step S314, where the focusfactors are ordered in the focus profile according to theircorresponding image numbers. FIG. 5 illustrates an example focus profile502 (shown as a solid line) within the region of interest 402 of FIG. 4.The focus profile 502 of FIG. 5 is shown as a graph plotting the focusfactor on the left side y-axis for each image slice number on thex-axis.

In Step S318, the computer system 106 searches for a local extremum inthe focus profile (i.e., a local maximum or a local minimum), usingknown techniques. A focus profile may have more than one local extremum.For example, FIG. 5 illustrates, in addition to the example of the focusprofile 502, local maxima 504 and 506 identified as a result of StepS318 being performed.

In one example embodiment herein, the computer system 106 compares eachfocus factor to its neighbors iteratively, wherein neighbors are definedas the focus factors within a predetermined range of image numbers ofthe focus factor being evaluated during an iteration. If the focusfactor being evaluated is greater than the individual focus factors ofall of its neighbors, the focus factor being evaluated is designated alocal maximum; otherwise, it is not designated a local maximum.

In another example embodiment herein, the computer system 106 performs afirst derivative test to search for the local maximum of the focusprofile. A first derivative of the focus profile is calculated from thefocus profile (e.g., calculating a difference value at each image numberof the focus profile by subtracting the focus factor at one image numberfrom the focus factor of the next greater image number), and then thelocal maximum is identified as corresponding to the image number wherethe first derivative of the focus profile crosses zero from positive tonegative. For example, FIG. 5 illustrates a first derivative 508 (shownas a dot-dash line, with magnitude of the first derivative on the rightside y-axis) corresponding to the focus profile 502. Local maxima 504and 506 are identified as corresponding to first derivative zerocrossings 510 and 512, respectively, as a result of performing Step S318according to an example of the present embodiment.

In a further example embodiment herein, the focus profile is filtered,that is, smoothed, by a moving average before searching for a localmaximum in the focus profile. FIG. 5 illustrates a filtered (smoothed)focus profile 514 (shown as a dotted line) corresponding to the focusprofile 502. The size of the moving average sample window may be, forexample, three neighboring focus factors. For some focus profiles,smoothing may improve the accuracy of the local maximum search.

In Step S320, the computer system 106 identifies the image numbercorresponding to the local maximum identified in Step S318 and extractsthe image slice associated with that identified image number from thestack of tomosynthesis images for display in Step S322. The extractedimage slice is also referred to herein as a high-focus image, because ithas a greater focus factor (that is, it is in greater focus) than othernearby image slices, as determined in Step S318. In one exampleembodiment herein, the high-focus image is extracted and stored in themain memory 232, although it may be stored instead in the secondarymemory 234, and can be retrieved therefrom for display in Step S322.

In the case where more than one local maximum is found in Step S318,more than one high-focus image corresponding to those local maxima areextracted. In an example embodiment herein, the high-focus images showone or more sub-object(s) 52 with clarity, by virtue of the processingperformed in Steps S314, S316, and S318, even though the sub-object(s)52 may not be visible in the conventional orthogonal projection image.This is because the presence of in-focus sub-object(s) 52 (e.g.,anatomical features) in a high-focus image generally corresponds to ahigh mean variance in the region of interest of that high-focus image,and the high mean variance can be identified as a local maximum in StepS320. For example, FIGS. 6 and 7 illustrate examples of image slicesextracted by performing Step S320. First, local maxima 502 and 504 inFIG. 5 are identified as corresponding to image numbers 7 and 36 in thetomosynthesis stack. In FIGS. 6 and 7, which correspond to image numbers7 and 36, respectively, clearly defined root structures 602 and 702 canbe seen, where those structures appear more clearly than in theorthogonal projection image shown on FIG. 4.

In Step S322, the computer system 106 displays the high-focus image(s)extracted in Step S320 on the display unit 108. In one exampleembodiment herein, the display unit 108 displays a composite imagecomprising a portion of the high-focus image extracted in Step S320,overlaid on the orthogonal projection image extracted in Step S308. Thecomposite image is formed by using the region of interest received inStep S312 to select a corresponding portion of the high-focus imageextracted in Step S320. The portion of the high-focus image is thenoverlaid on the region of interest indicated in Step S312 of theorthogonal projection image extracted in Step S308. For example, in theexample embodiment described above in Step S304 where each image sliceand the orthogonal projection image have the same dimensions, position,and orientation, the region of interest indicated in Step S312corresponds to the same pixel locations in both the high-focus imagesextracted in Step S320 and the orthogonal projection image extracted inStep S308. In the case where more than one high-focus image is extractedin Step S320, more than one composite image can be displayed in StepS322. For example, FIGS. 8 and 9 illustrate examples of the orthogonalprojection image shown in FIG. 4 with the region of interest portionsfrom the high-focus images of FIGS. 6 and 7, respectively, overlaidthereon.

In another example embodiment herein, displaying the high-focus image(s)in Step S322 can also include displaying a user interface on displayunit 108, with which the user may interact via input unit 114 and/ordisplay unit 108. FIG. 10 illustrates an example of such a userinterface 1000. The user interface 1000 can display an orthogonalprojection image in an orthogonal projection image viewport 1002. Aregion of interest 1006 can be visually depicted in the orthogonalprojection image viewport 1002 and can be the same region of interestobtained from the user in Step S312.

The user interface 1000 can also display, in a tomosynthesis imageviewport 1004, the entire stack of tomosynthesis image slices, or one ormore selected image slices of the stack. Stack controls 1016 can beprovided to enable the user to manually select, via a scroll bar forexample, which image slice of the tomosynthesis stack to display on thetomosynthesis image viewport 1004. In another example embodiment herein,the stack controls 1016 may include selectable control items, such as,for example, play, pause, skip forward, and skip backward, (not shown),or the like, to enable the user to control automatic display of thetomosynthesis stack, as a cine loop for example, on the tomosynthesisimage viewport 1004. An image location indicator 1018 also can beprovided to indicate to where in object 50 along the z-axis relative tothe x-ray detector 102 the image slice appearing in the tomosynthesisimage viewport 1004 corresponds. The location along the z-axis of theimage slice appearing in the tomosynthesis image viewport 1004,represented by the image location indicator 1018, is known, because, asdescribed above, the location of each image slice in the stack oftomosynthesis image slices reconstructed in Step S304 was provided as aninput to the tomosynthesis image reconstruction process of that step. Inan alternative embodiment herein, the z-axis location represented by theimage location indicator 1018 can be calculated (e.g., by the computersystem 106 or the computer processor 222) by multiplying the imagenumber of the image appearing in the tomosynthesis image viewport 1004by the z-axis slice thickness, which is a function of the known geometryof the system (e.g., the scan angle 112).

The user interface 1000 can also include pictorial indicators to helpthe user navigate the stack of tomosynthesis image slices, such as, forexample, a representation of the imaged object 1008 (which is apictorial representation of object 50), an x-ray detector icon 1010(placed in relation to the representation of the imaged object 1008 toindicate the placement of x-ray detector 102 relative to object 50 inStep S302), and high-focus image indicator lines 1012 and 1014 withinthe imaged anatomy 1008 (which are pictorial representations thatindicate the z-axis location within object 50 of high-focus imagesidentified and extracted in Step S320).

The number of high-focus image indicator lines appearing on the userinterface 1000 corresponds with the number of high-focus imagesextracted in Step S320. For example, in FIG. 10, two high-focus imageindicator lines 1012 and 1014 are shown, representing two high-focusimages extracted in Step S320 and illustrated in the examples of FIGS. 6and 7, respectively. In one example embodiment herein, selecting ahigh-focus image indicator line will display the correspondinghigh-focus image identified and extracted in Step S320 in thetomosynthesis image viewport 1004.

The process of FIG. 3 ends at Step S324.

Clinician users are accustomed to reading conventional two-dimensionalx-ray images due to their long-established use in medicine, but 3Ddatasets may be less familiar and more difficult to analyze. It can beappreciated that automated extraction of information from a 3Dtomographic dataset, including the identification within the 3Dtomographic dataset of high-focus images that contain features ofinterest (e.g., anatomical features), and presentation of the extractedinformation to a user may be useful. A clinician user may find it moreintuitive to have a portion of the high-focus image, which shows thefeature of interest, overlaid on and within the context of thetwo-dimensional orthogonal projection image, with which they areaccustomed from long-established practice. Additionally, automatedpresentation of the extracted information can save the clinician userthe time and effort associated with manually scrolling through andanalyzing a large number of image slices in order to locate features ofinterest.

Reducing Image Reconstruction Artifacts

The above intraoral tomosynthesis system 100 will now be furtherdescribed in conjunction with FIG. 17A, which shows a flow diagram of aprocess according to an example embodiment herein for using a mask basedon a two-dimensional orthogonal projection image to guide a process forreducing image reconstruction artifacts in an intraoral tomosynthesisdataset.

Prior to starting the process, the x-ray detector 102 and x-ray source104 are aligned manually by a user to a starting position, as describedabove, in one example embodiment herein.

The process of FIG. 17A starts at Step S1700, and in Step S1702 theintraoral tomosynthesis system 100 acquires a plurality of projectionimages of object 50 over a scan angle 112 (which may be predetermined),including the orthogonal projection image, in the manner describedabove. For example, the x-ray source 104 is moved by the motorized stage116 and control circuitry 118 to different positions within the scanangle 112, and the computer system 106 controls the x-ray source 104 toemit x-rays 110 at each position. In one example embodiment herein,x-ray source 104 is scanned, by pivoting at a point along the z-axis,from −20° from the z-axis to +20° from the z-axis in evenly distributedincrements of 0.8° to provide 51 scan angles, including the 0° position,although this example is not limiting. The x-rays 110 then pass throughand are attenuated by the object 50 before being projected onto thex-ray detector 102. The x-ray detector 102 converts the x-rays 110 intoelectrical signals (either directly or indirectly, as described above)and provides the electrical signals to the computer system 106. Thecomputer system 106 processes the electrical signals collected at eachscan angle position to acquire the plurality of projection images, eachimage comprising an array of pixels. The image acquired with the x-raysource 104 at the 0° position is also referred to herein as anorthogonal projection image.

In one example embodiment herein, the color depth of each pixel value ofthe projection images may be 12-bit grayscale, and the dimensions of theprojection images correspond to the standard dental size of the x-raydetector 102, as described above. For example, a Size-2 detector mayproduce projection images that are approximately 1700×2400 pixels insize, a Size-1 detector may produce projection images that areapproximately 1300×2000 pixels in size, and a Size-0 detector mayproduce projection images that are approximately 1200×1600 pixels insize.

In Step S1704, the computer system 106 processes the plurality ofprojection images acquired in Step S302 using a reconstruction techniquein order to reconstruct a series of two-dimensional tomosynthesis imageslices and may also perform deblurring and other image enhancements, aswill be described further herein. Each reconstructed image slice is atomographic section of object 50 comprising an array of pixels (eachpixel being located at a pixel coordinate), that is, each image slicerepresents a cross-section of object 50 that is parallel to the x-yplane in which the receiving surface of the x-ray detector 102 extends,has a slice thickness along the z-axis, and is positioned at adifferent, respective location along the z-axis than other image slices.The slice thickness is a function of the reconstruction technique andaspects of the geometry of the system 100, including, primarily, thescan angle 112. For example, each image slice may have a slice thicknessof 0.5 mm by virtue of the geometry of the system 100 and thereconstruction technique. The desired location of each reconstructedimage slice along the z-axis is provided as an input to thereconstruction performed in Step S304 either as a pre-programmedparameter in computer system 106 or by user input via input unit 114and/or display unit 108. By example only, the computer system 106 can beinstructed to reconstruct, from the plurality of projection images, afirst image slice that is one millimeter (1 mm) away from the surface ofx-ray detector 102 along the z-axis, a last image slice being at fifteenmillimeters (15 mm) away from the surface of the x-ray detector 102, andimage slices between the first image slice and the last image slice atregular increments along the z-axis of two-hundred micrometers (200 μm),for a total of seventy-one image slices.

Reconstruction of the tomosynthesis image slices in Step S1704 may beperformed in accordance with any existing or later developedreconstruction technique. In one example embodiment herein,reconstruction of the tomosynthesis image slices in Step S1704 utilizesa shift-and-add technique, described above. The shift-and-add techniqueutilizes information about the depth of sub-object(s) 52 along thez-axis that is reflected in the parallax captured by the plurality ofprojection images, as described above. According to this exampleembodiment, an image slice is reconstructed by first spatially shiftingeach projection image by an amount that is geometrically related to thedistance between the image slice and the tomographic focal spot 122along the z-axis. The shifted projection images are then averagedtogether to result in the image slice, where all sub-objects 52 in theplane of the image slice are in focus and sub-objects 52 outside of thatplane are out of focus and blurry. This shift-and-add process isrepeated for each image slice to be reconstructed. In the case of theimage slice corresponding to the x-y plane that includes the tomographicfocal spot 122, the projection images are averaged together withoutfirst shifting because sub-objects 52 are already in focus for thatplane.

The foregoing describes a basic shift-and-add reconstruction technique.In one example embodiment herein, a deblurring technique thatsubstantially reduces or removes blurry, out-of-plane sub-objects froman image slice can be performed in conjunction with the reconstructiontechnique (whether shift-and-add or another technique). Examples ofdeblurring techniques that can be employed include, for example, spatialfrequency filtering, ectomography, filtered backprojection, selectiveplane removal, iterative restoration, and matrix inversiontomosynthesis, each of which may be used in Step S1704 to deblur imagesreconstructed by the shift-and-add reconstruction technique (or anotherreconstruction technique, if employed).

In another example embodiment herein, Step S1704 also can include thecomputer system 106 performing further automated image enhancements suchas, for example, image blurring, image sharpening, brightnessoptimization, and/or contrast optimization, on each reconstructed (anddeblurred, where deblurring is performed) image slice in a known manner.

Additionally, in another example embodiment herein, the dimensions,position, and orientation of each image slice reconstructed in StepS1704 are the same as the corresponding characteristics of theorthogonal projection image. Thus, when tomosynthesis image slices (orportions thereof) and the orthogonal projection image are overlaid overone another, corresponding anatomical features appearing in the imageswill be overlapped and aligned without scaling, rotation, or othertransformation of the images.

After Step S1704, control passes to Step S1716, which will be describedbelow. Before describing that step, Steps S1706, S1708, and S1714 willfirst be described. Like Step S1704, Step S1706 is performed after StepS1702 is performed. (However, in other embodiments herein, Step S1704 isperformed serially between Steps S1702 and S1706, and then the methodcontinues from Step S1706 as shown in FIG. 17A. That is, control passesfrom Step S1702 to Step S1704, and then from Step S1704 to Step S1706,in which case, control does not pass from Step S1704 to Step S1716 asshown in FIG. 17A. In yet other example embodiments herein, Step S1704is performed serially between Steps S1714 and Step S1716, instead of theorder shown in FIG. 17A. That is, control passes from Step S1702 to StepS1706, at which point the method is performed until Step S1714 accordingto FIG. 17A, then control passes from Step S1714 to Step S1704, and thencontrol passes from Step S1704 to Step S1716, but control does not passfrom Step S1702 to Step S1704 as shown in FIG. 17A.)

Referring again to the embodiment represented in FIG. 17A, in StepS1706, the orthogonal projection image is extracted from the pluralityof projection images acquired in Step S1702. Because, as describedabove, the orthogonal projection image is defined as the projectionimage captured while the x-ray source 104 is in the 0° scan angleposition, no reconstruction is necessary to extract that image. In oneexample embodiment herein, the orthogonal projection image is extractedand stored in the main memory 232, although it may be stored instead inthe secondary memory 234. For example, FIG. 18 illustrates an exampleorthogonal projection image.

In Step S1708, the orthogonal projection image extracted in Step S1706is received as input and processed to generate as output a verified listof pixel coordinates of the orthogonal projection image that do notcontain a predetermined type of information, e.g., anatomic ordiagnostic information (such regions are hereinafter referred to as“non-anatomic regions” for convenience). Non-anatomic regions correspondto, in the imaged volume, air gaps, air pockets, and/or a mechanism forholding the x-ray detector 102, for example.

Step S1708 can be performed in different ways according to variousexample embodiments herein, including, but not limited to, either of thesubprocesses illustrated in FIGS. 17B and 17C. The subprocesses shown inFIGS. 17B and 17C will now be described in turn.

In one example embodiment herein, Step S1708 (FIG. 17A) may be performedaccording to the subprocess illustrated in FIG. 17B. The subprocess ofFIG. 17B starts in Step S1730, object edges within the orthogonalprojection image extracted in Step S1706 are detected by deriving atleast one edge map from the orthogonal projection image and comparingthe edge map to a threshold, in a manner described below. The edge mapcan be a variance image, a gradient image, or a combination thereof,and, in one example, the variance image and the gradient image can bederived by applying, respectively, a variance kernel operator and agradient kernel operator to the orthogonal projection image extracted inStep S1706 in a manner described in greater detail below. Thesensitivity of the variance and gradient kernel operators to edges, andthe width of the detected edges on the variance and gradient images, canbe adjusted by tuning the parameters of the operators. Additionally,after deriving the edge map but prior to comparing the edge map to athreshold, Step S1730 may also include a sub step of closing incompleteedges within the edge map, as will be described further below. Objectedges detected in Step S1730 are characterized by a high spatialfrequency (i.e., a sharp transition) and are typically associated withboundaries of objects of interest in an image.

A variance image (mentioned above) can be derived in Step S1730 byiterating a variance kernel operator, for example, a 5×5 pixel matrix,through each pixel coordinate of the orthogonal projection image. Ateach iterative pixel coordinate, the statistical variance of pixelvalues of the orthogonal projection image within the variance kerneloperator is calculated, and the result is assigned to a correspondingpixel coordinate in the variance image. Because variance represents thedegree to which the value of a pixel varies from those of its neighbors,high spatial frequency regions, such as an object edge, have a highvariance, while smooth, low spatial frequency regions have a lowvariance. For example, FIG. 19 illustrates a variance image derived fromthe orthogonal projection image illustrated in FIG. 18.

A gradient image (mentioned above) can be derived in Step S1730 byiterating a gradient kernel operator through each pixel coordinate ofthe orthogonal projection image. At each iterative pixel coordinate, thegradient kernel operator is convolved with pixel values of theorthogonal projection image at that pixel coordinate, and the result isassigned to a corresponding pixel coordinate in the gradient image.Examples of well-known gradient kernel operators include Roberts,Prewitt, and Sobel operators.

Because the gradient represents the magnitude of directional change inpixel value at a given pixel relative to its neighbors, a region with agreater directional change in pixel value, such as an object edge, willhave a greater corresponding gradient value than a region with no changeor non-directional change. For example, FIG. 20 illustrates a gradientimage derived from the orthogonal projection image illustrated in FIG.18.

In one example embodiment herein, the variance image and the gradientimage may individually serve as edge maps, and may be derivedsimultaneously or sequentially. In yet other example embodiments herein,only one of the variance image and the gradient image serves as an edgemap. In a further example embodiment herein, the edge map is derived bycombining the variance image and the gradient image by way of a logicoperator (e.g., AND, OR, etc.).

After the edge map is derived in Step S1730, Step S1730 may, in oneexample embodiment herein, further include a substep of closingincomplete edges within the edge map (also known as edge linking). Theedge map may have incomplete edges due to a number of factors,including, for example, noise or discontinuities in the pixel values ofthe orthogonal projection image, the parameters of the variance andgradient kernels, and/or the threshold applied to the edge map.Incomplete edges can be closed according to mathematical techniquesand/or localized curve fitting methods.

Finally, edges are extracted in Step S1730 by identifying pixels in theedge map that are greater than a threshold. The value of the thresholdmay depend on whether the edge map corresponds to the variance image,the gradient image, or a logical combination thereof. Furthermore,adjusting the threshold allows for control over the sensitivity to edgesand the width of the detected edges, and thus it also is within thescope of the method of FIGS. 17A and 17B to employ an adjustablethreshold in the edge extraction, wherein the threshold is adjustedbased on predetermined criteria over iterations of edge extractions.

After Step S1730 is performed, control passes to Step S1734, which willbe described below. Before describing that step, Step S1732 will firstbe described. Like Step S1730, Step S1732 is performed after Step S1706(of FIG. 17A) is performed. (However, in other embodiments herein, StepsS1730 and S1732 can be performed serially. For example, instead of theorder shown in FIG. 17B, in another example embodiment herein, StepS1730 is performed before Step S1732 (i.e., in between steps S1706 andS1732), whereas in another example embodiment herein, Step S1732 isperformed before Step S1730 (i.e., in between steps S1706 and S1730)).

In Step S1732, image statistics of the orthogonal projection image(extracted in Step S1706) as a whole, including, for example, a meanpixel value, a median pixel value, a pixel value histogram, and a pixelvalue cumulative histogram, are calculated and used to estimate thelocations of non-anatomic regions of the orthogonal projection image. Asmentioned above, non-anatomic regions correspond to, in the imagedvolume, air gaps, air pockets, and/or a mechanism for holding the x-raydetector 102, for example. Because no anatomy is present in theseregions to absorb emitted x-rays 110, the detector 102 receives morex-rays 110 and outputs higher electrical signal in these regions, whichappear, for example, as black or very dark areas on the projectionimages acquired in Step S1702. Thus, on the pixel value histogram, thesenon-anatomic regions have high pixel values relative to the rest of theorthogonal projection image, and may further correspond to one or moredistinctive peaks at the high pixel values (such distinctive peaks inthe pixel value histogram will correspond to large gradient increases inthe pixel value cumulative histogram). The image statistics (e.g., themean pixel value, the median pixel value, and the pixel value cumulativehistogram) can be combined to determine a threshold point that separatespixel values corresponding to anatomy from pixel values corresponding tonon-anatomy. For example, the threshold point may be specified manuallyby a user (via input unit 114 and/or display unit 108) or automaticallyby computer system 106 by selecting a pixel value that is above themedian pixel value but below the distinctive peak(s). Based on thethreshold point, a range of non-anatomic pixel values is deemed tocorrespond to regions containing no anatomic information. Finally, StepS1732 generates as an output a list of unverified non-anatomic pixelcoordinates, which represents the estimated locations of non-anatomicregions of the orthogonal projection image, from the pixel coordinatesof the orthogonal projection image (from Step S1706) that have pixelvalues within the range of non-anatomic pixel values.

For example, FIG. 21 illustrates image statistics of the orthogonalprojection image of FIG. 18, including a pixel value histogram 2100, apixel value cumulative histogram 2102, a median of the cumulativehistogram 2104, a distinctive histogram peak at high pixel values 2106,and a large gradient increase in the cumulative histogram at high pixelvalues 2108. A threshold point 2110 is determined for the exampleillustrated in FIG. 21 to be a pixel value of approximately 3800 basedon the image statistics, and a range of non-anatomic pixel values ofapproximately 3800 and above is deemed to correspond to non-anatomicregions. Finally, Step S1732 outputs an unverified list (not shown) ofnon-anatomic pixel coordinates from the orthogonal projection image ofFIG. 18 that have pixel values equal to or greater than 3800.

In certain circumstances, the list of unverified non-anatomic pixelcoordinates generated in Step S1732 may imperfectly separate anatomyfrom non-anatomy because the threshold point determined in Step S1732 isbased solely on the pixel value statistical information over the entireorthogonal projection image. That is, it may occur that some regions ofthe orthogonal projection image having pixel values within the range ofnon-anatomic pixel values (selected in Step S1732 based on the thresholdpoint) do in-fact contain anatomic information.

Thus, in Step S1734, the computer system 106 checks which pixelcoordinates of the list of unverified non-anatomic pixel coordinatesgenerated in Step S1732 are inside a spatial boundary formed by theedges detected in Step S1730 in order to generate a list of verifiednon-anatomic pixel coordinates. In particular, pixel coordinates withinthe boundaries are deemed to be anatomic regions, while pixelcoordinates outside the boundaries are deemed to be non-anatomicregions. Accordingly, pixel coordinates determined to be inside theboundaries (i.e., anatomic regions) are removed from the list, and pixelcoordinates determined to be outside of the boundaries (i.e.,non-anatomic regions) remain on the list, thereby providing a list ofverified non-anatomic pixel coordinates that is provided to Step S1714(FIG. 17A).

In another example embodiment herein, Step S1708 may be performedaccording to the subprocess illustrated in FIG. 17C, which evaluateseach pixel coordinate of the orthogonal projection image extracted inStep S1706 according to a conditional statement that states that if apixel coordinate has a high pixel value and a low pixel variance, thenthat pixel coordinate contains substantially no anatomic information.

In particular, the subprocess of FIG. 17C iterates through each pixelcoordinate of the orthogonal projection image extracted in Step S1706,starting with a first pixel coordinate in Step S1740. Then, in StepS1742, the pixel variance at the starting pixel coordinate iscalculated, for example, by computing the statistical variance of pixelvalues of the orthogonal projection image within a variance kerneloperator (e.g., a 5×5 pixel matrix centered on the starting pixelcoordinate).

In decision block S1744, the computer system 106 evaluates whether thepixel variance calculated in Step S1742 is low (by comparing the pixelvariance to a pixel variance threshold, which may be predetermined) andwhether the pixel value at the pixel coordinate is high (by comparingthe pixel coordinate to a pixel value threshold, which may bepredetermined). If both conditions are satisfied in decision blockS1744, then, in Step S1746, the pixel coordinate is added to a verifiedlist of pixels deemed to contain substantially no anatomic informationbefore control passes to decision block S1748. If either condition isnot satisfied in decision block S1744, then control passes directly todecision block S1748.

A pixel coordinate satisfying both conditions is deemed to containsubstantially no anatomic information because of the followingpresumptions regarding pixel variances and pixel values of theorthogonal projection image. First, areas of the orthogonal projectionimage containing anatomic information generally exhibit at least amoderately high pixel variance, while areas not containing anatomicinformation (e.g., air gaps, air pockets, or an x-ray detector holder)generally exhibit a low pixel variance. Second, as explained above withrespect to Step S1732, pixel coordinates that contain no anatomicgenerally have high pixel values and appear black or very dark, becauseno anatomy is present to absorb the emitted x-rays 110. On the otherhand, pixel coordinates containing anatomic information generallyexhibit lower pixel values. Thus, if a pixel coordinate has both lowpixel variance and high pixel value, it is deemed likely to not containanatomic information.

In some example embodiments herein, the pixel value threshold used indecision block S1744 can be derived from image statistics of theorthogonal projection image extracted in Step S1706 as a whole, asdescribed above with respect to Step S1732 of the subprocess illustratedin FIG. 17B.

At decision block S1748 (after decision block S1744 if “No” at decisionblock S1744 or after Step S1746 if “Yes” at decision block S1744), thecomputer system 106 checks if the last pixel coordinate in theorthogonal projection image has been evaluated by the conditionalstatement at decision block S1744. If the last pixel coordinate has notbeen evaluated yet (“No” at decision block S1748), then the subprocesscontinues to Step S1750, where the computer system 106 iterates to thenext pixel coordinate and subsequently evaluates that next pixelcoordinate in the above described manner (i.e., performs Steps S1742,S1744, S1746 if applicable, and S1748). If the last pixel coordinate hasbeen evaluated (“Yes” at decision block S1748), then the subprocess ofFIG. 17C ends by passing control and the verified list of pixelscompiled in Step S1746 to Step S1714 (FIG. 17A).

It will be apparent to a person skilled in the relevant art(s) thatwhile the subprocess of FIG. 17C is explained above on a pixel-by-pixelbasis, the subprocess of FIG. 17C also can be performed by differentcomputational techniques, including, for example, evaluating theconditional statement for the whole orthogonal projection imageextracted in Step S1706 in a single matrix operation based on theorthogonal projection image and a variance image generated from theorthogonal projection image (e.g., a variance image can be derived asdescribed above at Step S1730).

Referring again to FIG. 17A, in Step S1714, a binary mask image, havingthe same dimensions as the orthogonal projection image, is formed basedon the list of verified non-anatomic pixel coordinates provided in StepS1708. Each verified pixel coordinate on the binary mask image isassigned a pixel value of one. All other pixel coordinates on the binarymask image are assigned a pixel value of zero. For example, FIG. 22illustrates an example binary mask image obtained in Step S1714 (andcorresponding to the orthogonal projection image of FIG. 4).

In Step S1716, each tomosynthesis image slice reconstructed in StepS1704 is masked by the binary mask image formed in Step S1714 to isolatethe regions with no anatomic information. In an example embodimentherein, Step S1716 is performed by multiplying the binary mask imagewith each tomosynthesis image slice reconstructed in Step S1704 toprovide a corresponding masked tomosynthesis image slice. For example,FIG. 23 illustrates a tomosynthesis image slice masked by the binarymask image of FIG. 22, resulting in an isolated nonanatomic region 2300.

In Step S1718, the pixel values in the isolated nonanatomic regions ofeach masked tomosynthesis image slice obtained in Step S1716 arecompressed to provide corresponding reduced-artifact tomosynthesis imageslices. In an example embodiment herein, such compression is achieved bymapping the pixel values in the isolated regions through an intensitytransformation function into different output pixel values. Theintensity transformation function may be, in one example, configured toassign very low contrast to pixel values in isolated regions like, forexample, the intensity transformation function illustrated by graph 2400of FIG. 24.

In addition to compressing pixel values in the isolated regions, StepS1718 may also include a further substep of applying an anatomic-regionintensity transformation function to increase the contrast in regionscontaining anatomic information, the anatomic-region intensitytransformation function being different from the intensitytransformation function applied to isolated regions. Examples ofanatomic-region intensity transformation functions include a linearintensity transformation function illustrated by graph 2402 of FIG. 24,a sinusoidal intensity transformation function illustrated by graph 2500of FIG. 25, and a logarithmic intensity transform function (not shown).In the case that the foregoing substep is performed, Step S1718 outputsreduced-artifact tomosynthesis image slices where the pixel values inisolated regions have been compressed and pixel values inanatomic-regions have been contrast-enhanced.

In Step S1720, the computer system 106 displays one or more of thereduced-artifact image slices obtained in Step S1718 (i.e. the slicescorresponding to respective tomosynthesis image slices reconstructed inStep S1704) on the display unit 108. In one example embodiment herein,the displaying can be performed so as to show all reduced-artifact imageslices, or one or more selected reduced-artifact image slices, usingdisplay unit 108, and interactive controls are provided (e.g., viadisplay unit 108 and/or input device 114) to enable a user to selectbetween those two options, and to select one or more reduced-artifactimage slices for display, and also to select one or more particularregions of interest in the reduced-artifact image slice(s) for display(whether in zoom or non-zoom, or reduced fashion). For example, FIG. 26illustrates an example reduced-artifact image slice that may bedisplayed on the display unit 108.

In other example embodiments herein, reduced-artifact image slicesobtained in Step S318 may also be utilized in further processing methods(not shown) with or without being displayed in Step S320. In one exampleembodiment herein, the reduced-artifact image slices can be utilized togenerate clinical information from a tomosynthesis dataset according tothe method described further herein below with reference to FIG. 2B. Inanother example embodiment herein, the reduced-artifact image slices canbe utilized to interactively extract features for intuitive presentationto a clinician user, as described above. In another embodiment example,the reduced-artifact image slices can be utilized to render a 3D imageof object 50 and/or sub-object(s) 52. In a further embodiment exampleherein, the reduced-artifact image slices can be utilized for measuringdistances between points or structures of object 50 and/or sub-object(s)52.

The process of FIG. 17A ends at Step S1722.

Three-Dimensional Image Rendering from Image Slices

FIGS. 27A and 27B are flow diagrams of processes for rendering athree-dimensional (3D) image from tomosynthesis slices according toexample embodiments herein, or a sequence that can be used to enablesuch rendering.

Briefly, in FIGS. 27A and 27B, a series of two-dimensional x-rayprojection images over a limited scan angle are obtained. Tomosynthesisslices are reconstructed from the x-ray projection images. Aregion-of-interest is defined in one or more of the tomosynthesisslices. An outline trace of one or more objects in theregion-of-interest is created for each of the slices. Following this, avolume to be rendered may be generated by steps illustrated in eitherFIG. 27A or 27B, as discussed in more detail below.

As shown in FIGS. 27A and 27B, in step 2701, x-ray projection images areacquired over a limited scan angle. As discussed above, at each imagingposition within the scan angle, x-ray source 104 emits x-rays 110 whichpass through object 50 and are detected by the x-ray detector 102. Foreach imaging position, a projection image is obtained by the computersystem 106 based on the intensity of the x-rays received by x-raydetector 102. Thus, the system 100 collects a plurality of projectionimages (also referred to herein as “projections”) by positioning thex-ray source 104 at different angles, including, for example, the 0°position, and emitting x-rays 110 at each of those different anglesthrough object 50 towards x-ray detector 102.

In step 2702 of FIGS. 27A and 27B, a series of tomosynthesis slices arereconstructed by computer system 106 from the plurality of projectionsimages to form a tomosynthesis image stack. Each tomosynthesis slice isparallel to a plane in which the front surface of the x-ray detector 102extends and at a particular depth in the z-axis direction correspondingto that tomosynthesis slice. Image reconstruction may be accomplished byseveral different reconstruction techniques including, for example, ashift-and-add method, filtered backprojection, matrix inversiontomosynthesis, generalized filtered backprojection, SIRT (simultaneousiterative reconstruction technique), or algebraic technique, amongothers.

In an exemplary embodiment, a shift-and-add method may be used. Asdiscussed, the shift-and-add method takes into account the fact thatobjects at different heights (relative to the detector) will undergovarying degrees of parallax when exposed to x-rays at varying angles. Todevelop an image of objects at a specific height (i.e., a reconstructedtomosynthesis slice corresponding to a specific height), each projectionimage is shifted and added together with the other projection images atthat height from different angles such that all objects in a plane atthat height are in focus and objects outside of that plane are out offocus.

In step 2703 of FIGS. 27A and 27B, the reconstructed tomosynthesisslices obtained in step 2702 are preprocessed. The tomosynthesis slicesmay be preprocessed by undergoing one or more of edge enhancement, noisereduction filtering, and edge detection filtering. The preprocessingsteps will be described more fully below with respect to FIG. 28.

In step 2704 of FIGS. 27A and 27B, a region-of-interest in at least oneof the slices that underwent preprocessing in step 2703 is selected for3D rendering. In one example, an operator or user (such as a dentist)can operate an input unit (e.g., input unit 114) to define aregion-of-interest in one or more of the tomosynthesis slices in thetomosynthesis image stack, while a display unit (e.g., display unit 108)displays the slice and the current region-of-interest. For example, anoperator can drag a mouse to select a rectangle corresponding to theregion-of-interest on a displayed 2-D tomosynthesis slice, as shown inFIG. 29 and described more fully below. Of course, the invention is notlimited to this procedure for selecting a region-of-interest. In anotherexample, the region could be traced on a display screen, selected byentering coordinates, or could be automatically selected based onpredefined criteria.

As shown in FIG. 27A, after the region-of-interest is selected, thevolume gradient for the region-of-interest, in directions parallel tothe plane of the sensor, is measured in step 2719. From the measuredvolume gradient in step 2719, a three-dimensional outline trace of theobject may be determined (step 2720) by a variety of techniques,including a gradient segmentation technique or a minimal path (geodesic)technique, as described below and illustrated in FIGS. 27C and 27D.

FIG. 27C illustrates exemplary steps in determining the outline traceusing a gradient segmentation technique. In a gradient segmentationtechnique, the image volume is segmented (i.e., divided into sections)based on the magnitude of the gradient, and the output of the gradientsegmentation is a binary image. The gradient is segmented to identifyregions with a gradient magnitude (i.e., the absolute value of thegradient) above a certain threshold. Regions which are above thethreshold are assigned one binary value (e.g., 1) and regions below thethreshold are assigned another binary value (e.g., 0), resulting in abinary image. In step 2721, the measured volume gradient is processed asdescribed above to produce a binary image which is then analyzed todetermine connected edges, i.e., places where regions of differentbinary values are adjacent to one another. By determining the connectededges, separate objects may be identified and segmented from oneanother.

Once the objects are segmented, one or more object metrics such as thecentroid, length, gradient direction, orientation, and root edgeposition relative to the region-of-interest may be calculated for eachobject. Based on one or more of these metrics, in step 2722, a fuzzylogic algorithm tailored to a specific root geometry for a tooth may beused to determine which edge of the connected edges of a segmentedobject is most likely to be a given part of the tooth. The specific rootgeometry may be selected from a library of root geometries stored in acomputer. Fuzzy logic is a many-valued logic used to evaluateapproximate reasoning. In this embodiment, fuzzy variables have valuesbetween zero (0) and one (1), inclusive. A value of zero (0) correspondsto false. A value of one (1) corresponds to true. Values between zero(0) and one (1) represent a probability of being true. For each relevantobject metric, an acceptable range of values is identified and a measureof trueness is determined. For object metrics that are known to occupy afixed range, for example, root edge position relative to theregion-of-interest, the related fuzzy variable is zero outside of therange and a nonzero isosceles triangle within the range. Morespecifically, the probability increases linearly from one end of therange until a maximum probability is reached in the middle of the range,after which the probability linearly decreases until the end of therange is reached; thus forming an isosceles triangle with a maximumprobability in the middle of the variable range. For object metrics thathave a minimum value, the related fuzzy variable is zero below theminimum value and increases linearly until the object metric hits adefined saturation value. Each fuzzy variable corresponds to theprobability that an object with a given metric value corresponds to agiven portion of the tooth. The object metrics that are used are themaximum height, minimum height, the height, and the width of the object.To identify a given feature, the fuzzy variables are multiplied and thehighest measure is selected. Alternately, a neural network may be usedto identify the edges most likely to belong to the object.

In an alternative embodiment of embodiment 1, the outline trace iscalculated using geodesic approach, that is by determining the quickesttravel time between two points on a curved surface. First, a potentialvolume is estimated from the gradient volume calculated in step 2719. Inan exemplary embodiment, the potential volume may be estimated byselecting either positive or negative gradient values in the directionof source travel (step 2721A). Each value in the gradient volume may becompared to a threshold value to identify regions that have relativelyhigh gradient values, either positive or negative. Either positive ornegative gradient values are selected, and the non-selected oppositegradient values are set to zero in step 2722A. To avoid unreal valueswhen the thresholded gradient volume is inverted, an offset is added tothe zero values in step S2723A. The amount of offset is chosen to reducethe ratio of the highest and lowest values in the estimated travel timeto below a predetermined value (e.g., 10:1 ratio between highest andlowest values). The thresholded gradient volume is then inverted andsquared (S2724A) to produce a potential volume.

Once the potential volume has been calculated, paths of minimal travelare determined to identify edges of objects (S2725A). One exemplarytechnique for calculating the paths of minimal travel is a fast marchmethod, however, other techniques may also be used. In a fast marchmethod, a series of calculations are performed to determine the quickesttravel time from a starting point to one of the adjacent points (pointA). Then, another series of calculations are performed to determine thequickest travel time from point A to one of the adjacent points (pointB). This process repeats until the travel path reaches the desiredpoint. Here, the quickest travel path coincides with an edge of theobject and thus may be used to identify the edges (as opposed to thetechniques discussed above for steps 2721 and 2722). This technique hasthe advantage of automatically identifying three-dimensional branchesand being insensitive to small breaks in an edge. In a case where anyspurious edges (i.e., edges that are not part of the tooth) areidentified, those edges may be eliminated by reference to a nearbyvolume intensity, edge shape, or a known tooth topology. For example, anearby volume intensity may be used to determine a difference inintensities between volumes on either side of the identified edge. Ifthe edge corresponds to a tooth, then the difference in intensitiesshould be above a certain threshold. If the difference is below thatthreshold, then the edge likely does not correspond to a tooth's edgeand may be considered a spurious edge. In another example, if anidentified edge bends abruptly then it is unlikely to be part of a realtooth. Furthermore, if the topology of the tooth is known then theidentified edge may be compared to the known topology to determinewhether the edge corresponds to the tooth.

Once the outline trace is determined using either of the techniquesdiscussed above for embodiment 1, the outline trace is matched tocorresponding points on a reference object model in step 2726. Thereference model object is a surface model of a representative objectsuch as, for example, a molar with an appropriate number of roots. Thereference model object may be obtained either from available digitaldatabases or may be generated either by means of a commercialthree-dimensional optical scanner or by means of an x-ray tomographysystem. Although the outline trace determines the spatial position ofthe edges of the object, the depth of the object may not be determinedby the outline trace, as it is not visible in the limited anglereconstruction. The position in a depth direction of the front and backfaces of the object are estimated based on the ratio of the width of thereference object model to the thickness of the reference object model(aspect ratio). Positions of points on the front and back of the objectsurface are determined using the reference object model aspect ratio andthe width of the outline trace, so as to generate an estimated objectwith a similar aspect ratio to that of the reference object. In step2727, a morphing transformation based on the relationship between themodel object and the estimated object may be applied to the modelsurface to generate the estimated surface. A morphing transformation isa method of mapping one volume onto another volume in a continuousfashion. In an exemplary embodiment, a locally bounded three-dimensionalHardy transform may be used as a morphing transformation. Thethree-dimensional Hardy transform may be estimated based on thecorresponding points on the reference model object and the estimatedobject.

Next, in step 2728, the estimated surface created above is converted toa volume model and is reprojected into images matching the size anddimension of the original projections while assuming nominal attenuationvalues for dentin/enamel/pulp/air. In one embodiment, the estimatedsurface may be converted to a volume model first by assigning pointsadjacent to the estimated surface to the volume model and then fillingthe resulting hollow volume model. This may be accomplished usingprojection matrices calculated to match the projection geometry andusing a calculation technique similar or identical to techniques used toestimate the backprojection matrices used for reconstruction in order toensure the generation of compatible artifacts in the reprojected volume.Once the projection matrices are calculated, each projection matrix isthen multiplied by the volume model in a matrix multiplication operationto obtain a projection of the volume model. The number oftwo-dimensional projections corresponds to the number of projectionmatrices. The two-dimensional projection images obtained by reprojectingthe volume model obtained above (i.e., by simulating the passage ofx-rays through the volume model) are subtracted from the measuredprojections and the resulting images are inspected to determine whetheror not the estimated volume model is consistent with the data. Regionsof the two-dimensional projections that contain accurately calculatedobjects are expected to be substantially uniform and non-negative. Assuch, regions within the two-dimensional projections that aresubstantially non-uniform or negative indicate inaccuracy in the volumemodel. Such regions may be determined by a thresholding operation. Morespecifically, the difference between the calculated projection and therecorded projection may be calculated, and for a given pixel if thedifference exceeds a threshold value, that pixel is determined to be apoint of inaccuracy. The model surface is then adjusted by adjusting theposition of the extrapolated points (the points on the front and back ofthe object surface) in order to improve the consistency of the modelwith the measured data. For instance, if the remaining attenuation inthe subtracted projections is less than zero, the volume model thicknessdecreases. This is accomplished by local adjustment of the aspect ratioof the reference model used to calculate the model front and backsurfaces.

The technique described above, is also applied to the pulp chamber androot canal. In step 2714, the resulting images are displayed orotherwise presented.

Alternatively, in embodiment 2 (FIG. 27B), after a region-of-interest inat least one of the slices that underwent preprocessing in step 2703 isselected for 3D rendering, corresponding region-of-interests (“ROI”) arereceived by computer system 106 for each of the other slices in thetomosynthesis stack. In one example embodiment herein, the stack ofreconstructed slices can be searched to identify those that are in focusfor a region-of-interest selected in a single slice. As noted above, thetomosynthesis stack is composed of a series of tomosynthesis slices thatare two-dimensional images in respective planes that are parallel to thesurface of the x-ray detector. In an exemplary embodiment, a user mayscroll through the tomosynthesis stack. For example, a user can operatean input unit (e.g., input unit 114) to “scroll” or “page” through each,or selected ones, of the tomosynthesis slices in the tomosynthesisstack, while a display unit (e.g., display unit 108) displays thoseslices. To enable viewing of the slices, the displayed tomosynthesisstack can include a manual scroll bar and/or automation controls such asplay, pause, stop, rewind or the like. As such, if the user isinterested in focusing on a particular region-of-interest, or indetermining the depth placement of particular features in an image, theuser may interact with such functions to scroll through thetomosynthesis stack and visually evaluate each image or selected images(e.g., in order to designate a corresponding region-of-interest in eachof the images).

In step 2706, a loop is started to loop through each slice of the stackof N slices, e.g., slice “1” to slice “N”.

In step 2707, a “shrink-wrapping” algorithm is applied around thestructure in the current slice. Generally, a shrink-wrapping algorithmapproximates a surface by starting with a triangular mesh and deformingthe mesh to transform it to the required surface. A “shrink-wrapping”algorithm is a variant of an active contour/snake algorithm. An activecontour/snake algorithm operates by establishing a set of grid pointsthat form a contour enveloping the object and progressively evolving thepositions of the grid points subject to rules based on the imagecontent. For example, for a “shrink-wrapping” style operation, theinitial grid positions are placed outside of the object and thesepositions are evolved by rules assuming a repulsive “force” associatedwith the object and an attractive force between adjacent grid points.Thus, according to one example embodiment herein, the outline trace ofthe region-of-interest in each slice is created by applying ashrink-wrapping algorithm.

At the same time (or in another embodiment, not at the same time), instep 2708, an orthogonal input image (acquired in step 2701 as an x-rayprojection image captured while x-ray source 104 is in the 0° scan angleposition) is used to bound the shrink-wrapping algorithm being performedin step 2707. Thus, an orthogonal projection image is extracted from theplurality of projection images acquired in step 2701. Because, asdescribed above, an orthogonal projection image is defined as the x-rayprojection image captured while the x-ray source 104 is in the 0° scanangle position, no reconstruction is necessary or performed in step 2702to extract the orthogonal projection image. Accordingly, in each slice,the shrink wrapping is bounded by an orthogonal image of the object(s)in the region-of-interest received in step 2705.

In step 2709, an outline of the imaged object (i.e., object 50) isformed in each slice of the stack, based on the shrink-wrappingalgorithm bounded by the orthogonal image. Thus, for each slice, anoutline of the structure in the region-of-interest is obtained. Inaddition, in step 2709, each contour corresponds to a single slicethrough the volume. The surface of that volume is smoothed to eliminatenon-uniformities by, for example, Gaussian filtering of the volume or byLaplace flow smoothing of a surface mesh.

In step 2710, there is a determination, based on integrated edgestrength adjacent to the contour, of whether the last slice of theobject within the image stack has been processed in the loop. Morespecifically, the gradient magnitude is integrated along the edgecontour to estimate the overall edge strength. If not, the procedurereturns to step 2706 to process a next slice in the stack. On the otherhand, if the last slice has been processed, the procedure proceeds tostep 2711.

In step 2711, the outlines formed in step 2709 are up-sampled in thez-dimension, and linear or trilinear interpolation is performedin-between the outlines to provide a higher-resolution contour. Inparticular, the collection of outlines, e.g., the set comprising eachrespective outline for each slice in the stack obtained for theregion-of-interest in step 2709, represents a discretized contour of a3D surface, and interpolating between the outlines will provide ahighly-sampled and smooth 3D contour.

In step 2712, the nodes of neighboring outlines are aligned in thez-dimension. Any discontinuities in the volume are eliminated byaligning the final position of the grid points of the active contour ofneighboring outlines. This may be done by linearly interpolating thegrid positions between those estimated in adjacent slices.

In step 2713, the 3D surface contour is created from the alignedoutlines. In this manner, the output of this technique is a 3D renderingof the dataset which is subsequently displayed as shown, for example, asview 2904 in FIG. 29 or as view 3004 in FIG. 30, which will be discussedmore fully below.

In step 2714, the 3D surface contour obtained in step 2713 is displayedor otherwise presented. The 3D contour can, for example, be displayed ona display screen in a separate window, as shown by view 2904 in FIG. 29or view 3004 in FIG. 30. The display may be provided with lighting,rotational, camera, coloring, viewing controls for the 3D contour,and/or the like. Thus, for example, the contour can be rotated freelyand a full 3D visualization, including side and back views, of thestructures can be provided. In addition, measurement capabilities can beincorporated to provide true 3D measurements between multiplestructures/features, as described more fully below with respect to FIG.31A to FIG. 31C.

FIG. 28 is a flowchart (titled “Pre-Processing”) illustrating aprocedure for pre-processing reconstructed tomosynthesis slicesaccording to an example embodiment herein, and represents in furtherdetail the manner in which step 2703 in FIGS. 27A and 27B is performed.In step 2801, edge enhancement is performed for each slice reconstructedin step 2702, for example, by use of Fourier filter which ramps linearlyfrom 100% to 30% ranging from maximum spatial frequency to zero spatialfrequency. Thus, in one example herein, a filter is used to increase theedge contrast of each structure in the slice in order to improve itssharpness.

In step 2802, noise reduction filtering is performed on each of theslices, using, for example, a low-pass filter. Similarly, for example,each volume slice may be convolved with a Gaussian kernel with a radiusbetween 0.25 and 1 pixel in order to reduce noise. Accordingly, noiseand other artifacts can be removed or reduced from the slices.

In step 2803, edge detection filtering is performed on each slice, tomore clearly define the edges of each structure (e.g., object 50, suchas one or more teeth). Typically, edge detection filtering may beperformed in many manners. One technique to estimate the image gradientin a given direction is by convolution with a kernel estimated from thederivative of a Gaussian in the given direction. As a result of thepre-processing described with respect to FIG. 28, the visibility of theslices is enhanced.

FIG. 29 is a view for illustrating a display and user interfaceaccording to an example embodiment herein.

In particular, as shown in FIG. 29, multiple windows can be presented(via, e.g., display unit 108) to allow for convenient control andanalysis by a viewer.

In the example shown in FIG. 29, image 2901 is an image for defining aregion-of-interest 2906, as described above with respect to step 2704 ofFIGS. 27A and 27B. An example of a 3D rendering (of theregion-of-interest 2906) is shown in image 2904, and can be obtainedaccording to the procedure described in FIG. 27A.

As shown in FIG. 29, an image 2901 is an orthogonal projection imageobtained in step 2701, and corresponds to an image captured while thex-ray source 104 is in the 0° position shown in FIG. 1A above. Theorthogonal image 2901 can be displayed alongside an image 2902 of acurrent slice (as shown), in order to provide a reference view forcomparison. Thus, in one embodiment, the orthogonal projection image2901 is displayed as a default image regardless of whether the user hasselected another slice, and is displayed separately from any currentlyselected slice, and, so as to provide a visual default reference.

Image 2902 depicts a currently selected tomosynthesis slice in thestack. Specifically, image 2902 is a currently selected image from thetomosynthesis stack (which was reconstructed in step 2702 from theprojection images obtained in step 2701) that has undergonepreprocessing in step 2703. A user may change the displayedtomosynthesis slice 2902 by navigating to different slices in the stack.In particular, using an input unit (e.g., input unit 114), the user canview images at different planes in the stack. For example, in theembodiment shown in FIG. 29, the user can click or otherwise designatearrow indicators on scroll bar 2905 to toggle through each slice tonavigate to a desired slice. Scroll bar 2905 may also include additionalvisual indicators of the position of the currently displayed slice inthe stack. For example, in the embodiment shown in FIG. 29, scroll bar2905 displays the slice position relative to the detector plane for thecorresponding slice image 2902. The scroll bar may, for example,indicate whether the slice is towards a buccal or lingual position.

FIG. 30 is a view for illustrating another example display and userinterface according to an example embodiment herein. In FIG. 30, theorientation and function of images correspond generally to thosedepicted in FIG. 6, but after a rotation of the 3D rendering (see 3004).Thus, in FIG. 30, image 3001 corresponds to an image of the slice withthe region-of-interest remaining highlighted, image 3002 depicts thecurrent slice, image 3003 depicts the current position of the sensor,and scroll bar 3005 is for scrolling through the slices. Image 3004depicts the 3D rendering of the tooth and may be rotated (in anydirection) to allow additional 3D viewing of the structure of the objectof interest (e.g. a root). The example of a 3D rendering shown in image3004 can be obtained according to the procedure described in FIG. 27B.In one example embodiment, display in step 2714 is updated according touser input. In particular, the user can manipulate the 3D image in view3004 to orient the displayed object in the manner desired, by, forexample, accessing viewing controls or predefined input device actions(e.g., via input unit 114 shown in FIG. 1A), to thereby view differentaspects of the tooth (or other structure). Accordingly, the 3D image isdisplayed in step 2714 (e.g., via display unit 108 shown in FIG. 1A),and visual aspects of the 3D image can be manipulated to provide adifferent display.

FIGS. 31A to 31C are views for illustrating measurement of distances ina 3D image according to example embodiments herein.

Specifically, FIGS. 31A to 31C show different states of a 3D renderingimage of a tooth depicted on a display unit (e.g., via display unit 108shown in FIG. 1A), as a user sets measurement points and enablesmeasurement of distances on the 3D rendering.

Specifically, FIG. 31A is an illustration representing examples of how ameasurement process between two points on a 3D rendering of a tooth canbe started. As shown in image 3101, a point on the 3D rendering of atooth (e.g., using a mouse or other interface device) is navigated to.

In image 3102, with the desired first measure point “A” having beennavigated to, a user manipulates an input device (e.g., via input unit114 shown in FIG. 1A) to display a user interface (e.g., menu 3103), forexample by right-clicking on a mouse. Menu 3103 enables a user to, forexample, rotate or manipulate the 3D rendering, set mouse controls, sethot keys, obtain further information (“about”), and measure betweenpoints on the 3D rendering. In the example shown in image 3102, the userhas selected the “measure” option 3104, which brings up a further userinterface 3105 enabling the user to set a measure point A, a measurepoint B, to request the distance between A and B, and to reset. Asshown, the user selects “Set Point A” to set a first measurement point(indicated by the triangle in FIG. 31A).

Turning to FIG. 31B, the user follows a similar procedure to set themeasure point B. As can be seen from image 3106, however, the 3Drendering has been rotated from the views shown in FIG. 31A. Thus, image3106 represents an example of a case where the user has rotated andmanipulated the view for better access to measure points desired forselection. Once measure point B is selected (represented by the cross inFIG. 31B), image 3107 is provided with a line 3108 connecting the twopoints A and B.

In FIG. 31C, in image 3109, the user selects to calculate the distancebetween the points A and 13 that were selected as shown in FIG. 31A andFIG. 31B. This can be accomplished with, for example, the same drop-downmenu used to set the measurement points A and B. As a result, thedistance between points A and B is calculated. For example, assuming thecoordinates of point A in the 3D space are (x₁, y₁, z₁) and thecoordinates of point B in the 3D space are (x₂, y₂, z₂), the distancecan be calculated as d=√{square root over ((x₂−x₁)²+(y₂−y₁)²+(z₂−z₁)²)}.

In image 3110, a window 3111 appears, displaying the calculated distancebetween points A and B, along with coordinates of the points A and B inthe 3D space. Accordingly, the user is provided with a convenient toolto easily view and measure distances between structures (or distanceswithin structures) in the 3D rendering, whether the structures are inthe same image slice or different image slices.

As discussed, for x-ray images to have value and utility in clinicaldiagnosis and treatment, they should have high image fidelity andquality (as measured by resolution, brightness, contrast,signal-to-noise ratio, and the like, although these example metrics arenot limiting) so that anatomies of interest can be clearly identified,analyzed (e.g., analysis of shape, composition, disease progression,etc.), and distinguished from other surrounding anatomies. The processesdescribed in FIGS. 27A and 27B can be used to improve image fidelity andquality by reducing image artifacts and optimizing image contrast.

Measuring Three-Dimensional Distances in a Stack of Images

In accordance with an example aspect described herein, a method, system,apparatus, and computer program product are provided for measurementbetween structures in tomosynthesis images, and more particularly, formeasuring 3D distances in a stack of tomosynthesis images will now befurther described in conjunction with FIG. 32, which shows a flowdiagram of an exemplary process for measuring distances in athree-dimensional tomosynthesis images according to one exampleembodiment herein.

Briefly, by virtue of the procedure of FIG. 32 and as will be describedbelow, distances between points in a three-dimensional spacecorresponding to a tomosynthesis dataset may be obtained. A stack oftomosynthesis images (slices) is obtained. Specifically, the stack oftomosynthesis images is reconstructed from x-ray projection images takenover a scan angle. The tomosynthesis slices are a series of images whichare parallel to a plane defined by the surface of the x-ray detector102. The tomosynthesis stack is comprised of the tomosynthesis slices.Each tomosynthesis slice is stacked on top of or below another slice ina direction orthogonal to the plane of the x-ray detector. Thus, theplurality of tomosynthesis slices comprising the tomosynthesis stackcorrespond to an image volume (i.e., a certain volume of space inthree-dimensions). If the plane of the x-ray detector is defined as thex-y plane and a direction orthogonal to the plane of the x-ray detectoris defined as the z-axis, then a point P₂ within the image volume may beidentified by it three-dimensional Cartesian coordinates (x_(i), y_(i),z_(i)).

In the exemplary embodiment shown in FIG. 32, a distance from one pointin the image volume to another point in the image volume may bedetermined. Briefly, a first point on a first tomosynthesis slice and asecond point on a second tomosynthesis slice (or the first tomosynthesisslice) are selected. The computer system 106 may determine the x-ycoordinates of the first and second points from the pixel arrayscorresponding to the respective tomosynthesis slices. As noted above,each tomosynthesis slice occupies a certain position within thetomosynthesis stack. In other words each tomosynthesis slice lies acertain distance from the surface of the x-ray detector along thez-axis. Thus, all the points within a tomosynthesis slice have the samevalue for a z-coordinate. Thus, by knowing the system geometry and theposition of the tomosynthesis slice within the tomosynthesis stack, thez-coordinate may be determined.

As one of ordinary skill will appreciate, the x-y coordinates of a givenpoint may correspond to a particular anatomical plane. For example, atomosynthesis slice may lie in a plane substantially parallel to theocclusal surface. The relationship between the tomosynthesis slices andcertain anatomical features will be further explained below in referenceto FIGS. 39A-41B.

In step 3200, the computer system 106 generates a tomosynthesis stackcomprising a plurality of tomosynthesis slices from a plurality ofprojections images, as discussed above and with reference to FIGS.1A-1D.

In step 3201, a user selects a tomosynthesis slice from thetomosynthesis stack. In an exemplary embodiment, the user may operate aninput unit (e.g., input unit 114) to “scroll” or “page” through each, orselected ones, of the slices in the tomosynthesis stack, while a displayunit (e.g., display unit 108) displays those slices. To select aparticular slice, the user may stop scrolling or otherwise movingthrough the stack. An example embodiment illustrating this process isshown in FIGS. 33A-33D, which will be discussed more fully below.

Once the user has selected a particular slice, the user may, in step3202, use the input unit 114 to select a point on the displayed slice toplace a marker, which then serves a first measurement point. In anexemplary embodiment, this marker indicates a particular anatomicallocation or plane. The user may also change the location of the markerafter it is initially set. For example, the user may manipulate theinput unit 114 (e.g., a mouse) to move the marker over the displayedslice, and then use another input (e.g., clicking a button on the mouse)to designate the placement of the marker once the marker is in a desiredlocation (step 3203).

Once the marker is placed at the desired location, the location of themarker is set as a point A, and the x-y coordinates of point A are savedin memory as point A (or using some other designation). The marker forpoint A may be displayed differently from a marker in another laterselected slice for a point B, in order to help the viewer quicklyobserve that the points are in different slices and thus at differentpositions along the z-axis. For example, a marker for point A may bedisplayed in a different color than a marker for point B. An exampleembodiment illustrating this process is shown in FIGS. 34A-37D, whichwill be discussed more fully below.

Turning now to step 3207, before or concurrently with the actions insteps 3201 to 3203, a three-dimensional volumetric image (“volumetricimage”) of the image volume is created. The volumetric image may providefor better visualization and representation of an imaged object. Forexample, if the imaged object is a tooth, the volumetric image mayprovide a user with a perspective view of the tooth which aids indetermining a relative location of the slices. The volumetric image ofthe image volume may be generated from the tomosynthesis stack. Asdiscussed above, the tomosynthesis stack includes a plurality oftomosynthesis slices at a plurality of depths, and the volumetric imageof the image volume may be generated from the tomosynthesis slices. Inanother example embodiment herein, the volumetric image may representthe three-dimensional structure of the imaged anatomy or it mayrepresent a model volume that approximates the shape of the anatomy.

As discussed above, each tomosynthesis slice in the tomosynthesis stackcorresponds to a two-dimensional image in an x-y plane (parallel to theplane of the x-ray detector). The z-axis is orthogonal to the surface ofthe x-ray detector 102 and thus a coordinate value for the z-axisrepresents a distance from the surface of the x-ray detector 102. Thus,as discussed above, scrolling through the tomosynthesis slices,corresponds to traveling through the image volume in a direction towardsor away from the surface of the x-ray detector.

If the system geometry is known (or deduced from one or more objectswithin the plurality of projection images such as alignment markers) thecomputer system 106 may determine a distance between two tomosynthesisslices. For example, if a tomosynthesis slice lies just beyond thesurface of a tooth (e.g., proximate to but not including, for example, abuccal surface), the computer system 106 may identify that slice andproduce a depth measurement based on the separation between that sliceand another slice. For example, if a slice proximate to but notincluding a buccal surface of a tooth is labelled S_(i), and anotherslice located beneath, and within the tooth structure is labelled S_(j),then the depth of slice S_(j) relative to the buccal surface may becalculated based on the difference between i and j and the knowndistance between slices.

In step 3208, the marker placed at point A is transferred to acorresponding location on the volumetric image. The coordinates of pointA may also be set as a geometrical base (i.e., an origin) of ameasurement vector from point A to point B (discussed below), in theimage volume.

To determine point B (step 3205), a user may scroll through thetomosynthesis stack to a desired tomosynthesis slice (step 3204) andthen place a second marker (step 3206) at a desired location (point B)on the displayed tomosynthesis slice. The x-y coordinates of point B aresaved in memory under that designation (or some other designation). Asdiscussed above, a user may operate the input unit (e.g., input unit114) to scroll or page through each of the slices in the tomosynthesisstack, and the display unit (e.g., display unit 108) displays eachslice. As discussed, a user may (at least temporarily) pause or stopscrolling, toggling, or otherwise moving through the stack when the userreaches the desired slice. Typically, the second marker would be placedon a different tomosynthesis slice (S₂) from the tomosynthesis slicecontaining the first marker. If the second marker were placed on thesame slice as the first marker, then the measurement would correspond toa linear distance between the first and second marker within a planedefined by the tomosynthesis slice. In other words, a two-dimensionalmeasurement as opposed to a three-dimensional measurement. Nevertheless,it should be understood that the second marker, and thus the secondmeasurement point, could be in the same tomosynthesis slice as that ofpoint A.

The second marker may be visually distinguishable from the first marker.For example, the second marker may be a different color, shape, or sizefrom the first marker. Moreover, the first marker may remain visible onthe displayed tomosynthesis slice, even if the displayed tomosynthesisslice is different from the tomosynthesis slice in which the firstmarker is placed. The user may therefore see the x-y location of thefirst marker as the user scrolls through the tomosynthesis stack.

As the user operates the input unit 114 to place the marker for thesecond measurement point, the computer system 106 calculates themeasurement vector from point A to point B, which respectivelycorrespond to the first and second measurement points in the imagevolume (step 3209). If the user moves the second measurement point (orthe first measurement point) to a different location using the inputunit 114, the computer system 106 may dynamically update a themeasurement vector as the measurement point is moved. Thus, in oneembodiment, the computer system 106 may dynamically update themeasurement vector while either the first or second measurement point ismoved or changed, in addition to calculating the measurement vector whenthe two measurement points are initially set.

In a similar manner to step 3208, a point corresponding to point B isplaced on the volumetric image of the image volume generated in step3207 (step 3210). As discussed, the volumetric image provides the userwith a three dimensional view of the image volume. Thus, by placingpoints corresponding to points A and B, respectively, on the volumetricimage, the user may appreciate the three dimensional relationshipbetween points A and B, which may be not be obvious from thetwo-dimensional tomosynthesis slices.

In step 3211, the computer system 106 calculates the vector V_(AB)between points A and B. The vector includes both a magnitude anddirection. For example, assuming the coordinates of point A in the 3Dspace are (x₁, y₁, z₁) and the coordinates of point B in the 3D spaceare (x₂, y₂, z₂), the vector magnitude may be calculated according tod=√{square root over ((x₂−x₁)²+(y₂−y₁)², +(z₂−z₁)²)}. The computersystem 106 may also calculate the x, y, and z vector components. Thesevector components may also correspond to the mesiodistal, buccolingualand coronoapical axes as discussed below.

Of course, other methods or formulas for calculating the magnitude ofthe vector can be used in step 3211. The magnitude of the vector canthen be displayed on the display unit 108 in a variety of manners, asdiscussed more fully below. The features described above and illustratedin FIG. 32 will now be explained in further detail below with referenceto FIGS. 33A-41B.

FIG. 33A is a two-dimensional projection image 3301 recorded by thex-ray detector 102 at the 0° position, as shown in FIG. 1A above. Asdiscussed above, the plurality of projection images are used by computersystem 106 to create a tomosynthesis stack comprising a plurality oftomosynthesis slices. FIG. 33B illustrates the spatial relationshipbetween a tomosynthesis slice 3302 (shown in FIG. 33C) located adistance d₁ from the x-ray detector 102 in the z-axis direction. Thetomosynthesis slice is number S₁ of S_(i) comprising the tomosynthesisstack, as indicated by slice number display 3304 adjacent to scroll-bar3306. FIG. 33D is an illustration showing the location of thetomosynthesis slice 3302 within the image volume.

In an exemplary embodiment, one or more of the images shown in FIGS.33A-33D may be displayed at the same time on a display unit (e.g.,display unit 108), so that a user can conveniently view informationrelating to a current slice and its relative position.

As discussed above, a user may use an input unit 114 to change thedisplayed tomosynthesis slice. For example, the user may interact withscroll-bar 3306 to change the displayed tomosynthesis slice. The usermay drag the scroll-bar icon positioned on scroll bar 3306 to togglethrough each slice to navigate to a desired slice in the z-axisdirection. Scroll bar 3306 may also include additional visual indicatorsof the position of the current slice in the stack. For example, a textbox may be displayed concurrently with the tomosynthesis slice thatshows a distance (d_(i)) from the tomosynthesis slice to the surface ofthe x-ray detector 102. As the user scrolls through the tomosynthesisstack, the value d_(i) may be concurrently updated to correspond to thedisplayed tomosynthesis image.

As discussed above in regard to steps 3202 and 3203, once a user selectsa desired tomosynthesis slice, a marker 3402 may be placed at a desiredlocation, as illustrated in FIG. 34. In one embodiment, the user mayplace the marker 3402 by a double-tap operation of a mouse, touchscreen,or similar device. In another embodiment, the user may use the input 114to bring up a menu graphical user interface menu (“menu”) that mayinclude a command “Set As Point A,” along with other commands (e.g.,“Set As Point B”, “Clear Point A”, “Clear Point B”, “Clear All Points”,“Rotate Measurement Axis”, “Reset 3D View”). Some of the commands may be“greyed out”, i.e., unavailable, during the process illustrated in FIG.32. By selecting the command “Set As Point A”, the desired location maybe set as point A, and a marker 3402 may be placed at that location.

As discussed above in reference to steps 3204-3206, once point A isselected, the user may select another point (point B) on the sametomosynthesis slice or a different tomosynthesis slice. As discussedabove, the user may use input unit 114 to navigate to the desiredtomosynthesis slice (if applicable). FIG. 35C is an illustration of asecond tomosynthesis slice 3502 (S₂) located a distance d₂ from thex-ray detector 102 (as illustrated in FIGS. 613 and 6D) which the userhas navigated to. As shown in FIG. 35C, the slice number display 3304now reads S₂. FIG. 35A is the x-ray projection image recorded at the 0°position and is identical to FIG. 33A, discussed above. As discussedabove, one or more of these images may be displayed on display unit 108,either concurrently or individually. While FIGS. 35B-35D illustrate atomosynthesis slice 3502 which is further away from the surface of thex-ray detector 102 in the z-axis direction, the second tomosynthesisslice could also be located closer to the x-ray detector 102. Asdiscussed, a text box may also be displayed on the display unit 108which indicates the distance from the selected tomosynthesis slice 3502to the x-ray detector 102. It should be noted, that the distance fromthe tomosynthesis slice 3502 to the x-ray detector 102 is only one typeof distance that could be displayed. If, as discussed above, thecomputer system 106 determines that a particular tomosynthesis slicecorresponds to a position proximate to, but not including, a surface ofthe imaged object, a distance from that tomosynthesis slice to any othertomosynthesis slice may also be calculated and displayed (either aloneor along with another distance). Such a distance calculation may providean approximate depth from the surface of the imaged object to thedisplayed tomosynthesis slice.

As shown in FIG. 35C, the marker 3402 placed at point A may be visiblein the displayed tomosynthesis slice 3502. In an exemplary embodiment,the marker 3402 may be displayed differently when a differenttomosynthesis slice is displayed. For example, as shown in FIG. 35C,marker 3402 is filled with dashed lines which indicate that the marker3402 was placed in a different tomosynthesis slice from the onedisplayed (tomosynthesis slice 3502). Alternatively, one or more of thesize, shape, color, fill pattern, and transparency of marker 3402 mayalso be changed.

FIG. 36 is an enlarged view of tomosynthesis slice 3502 shown in FIG.6C. As discussed above, a user may place a second marker 3602 at adesired location. In the example shown in FIG. 36, the second marker3602 is a triangular shape to distinguish it from the first marker 3302.However, the markers 3402 and 3602 may be the same shape, but havedifferent colors or sizes to distinguish one from the other. The markers3402 and 3602 may also have different fill patterns or transparencies todistinguish one from the other. As discussed, the second marker 3602 maybe set through an input from the input unit 114. For example, if theuser is using a mouse, the user may right-click to bring up a menu (asdiscussed above) that includes one or more commands, such as “Set AsPoint A”, “Set As Point B”, “Clear Point A”, “Clear Point B”, “Clear AllPoints”, “Rotate Measurement Axis”, “Reset 3D View”.

As shown in FIGS. 37A-37D, once the second marker 3602 is set, thevector (V_(ab)) 3702 may be calculated from the first marker 3402 to thesecond marker 3602 and displayed three-dimensionally in a coordinatesystem representing the image volume (see FIG. 37D). As discussed above,the vector magnitude 3704 (D_(ab)) may be calculated and displayed aswell.

In an exemplary embodiment, however, the user does not have to set thesecond marker 3602 in order to calculate and display a correspondingvector. Rather, as the user moves the second marker 3602 over thedisplayed tomosynthesis slice 3502, the computer system 106 dynamicallytracks the movement of the second marker 3602 and produces the vectormagnitude 3704 and the displayed vector 3702 between points A and B onthe volumetric view of the image volume (see FIG. 37D). As discussed infurther detail below, depending upon the type of diagnostic image andthe selected points, the displayed vector 3702 may correspond to certainanatomical measurements, such as, for example a mesiodistal length,coronoapical length, and buccolingual length

Accordingly, even without setting the second marker 3602 at the secondmeasurement point (point B), the user is provided with the vectormagnitude and a visual depiction of the vector from the firstmeasurement point (point A) represented by the first marker 3402 to thesecond measurement point (point B) represented by the second marker3602. As such, the vector is dynamically updated as the second marker ismoved to a new location.

In an exemplary embodiment, the volumetric view of the image volume maybe rotated to provide the user with a different viewing angle, asillustrated in FIGS. 38A and 38B. FIG. 38A is identical to FIG. 37D,which is a default viewing angle for the volumetric view. FIG. 38B isanother volumetric view of the image volume, which has been rotated toprovide an additional perspective view. The particular perspective viewshown in FIG. 38B is only exemplary. The image volume (and thecalculated vectors displayed therein) may be viewed from any angle orperspective. In addition, the user may select from a GUI menu one ormore preset views, such as, for example, an X-Y view, an X-Z view, and aY-Z view. Therefore, according to the example embodiment herein, a viewof the three-dimensional coordinate space may be rotated.

In an exemplary embodiment, a user may quickly navigate through thetomosynthesis stack using one or more of the markers. As discussed, thecomputer system 106 may cause the display unit 108 to change thedisplayed tomosynthesis slice based on a command received through theinput unit 114. If a user selects a marker corresponding to atomosynthesis slice which is not currently displayed, the user may entera command through the input unit 114 which will cause the computersystem 106 to retrieve and display the corresponding tomosynthesisslice. The user may select the marker by, for example, double-clickingon the marker, or by right-clicking on the marker to bring up GUI menusystem, and then selecting an appropriate command, such “DisplayCorresponding Tomosynthesis Slice”. For example, if tomosynthesis slice3502 is currently displayed (as shown in FIG. 37C), the user maydouble-click on the first marker 3402 (corresponding to tomosynthesisslice 3302), and the computer system 106 will then cause the displayunit 108 to display tomosynthesis slice 3302 (see FIG. 33C). The slicenumber display 3304 will then display the corresponding tomosynthesisslice number (e.g., S,). As such, it is possible for a user to quicklyreturn to a view where a marker was originally set, or other views whereother markers were set.

While the above description has detailed the placement of first andsecond markers, more markers may be placed, either on the sametomosynthesis slice or other tomosynthesis slices. The computer system106 may, in a manner similar to that described above, calculate anddisplay additional vectors corresponding to these markers on displayunit 108. The respective magnitudes of these vectors may also bedisplayed (like in FIG. 37D), either concurrently with other vectormagnitudes or individually.

FIGS. 39A-41B illustrate another exemplary embodiment. FIGS. 39A-39C areillustrative of three images which may be simultaneously or individuallydisplayed on display unit 108. In FIG. 39A, a two-dimensional x-rayprojection image 3902 corresponding to the 0° angle shown in FIG. 1A isdisplayed on display unit 108. In the manner discussed above, a user mayselect a tomosynthesis slice 3904 from the tomosynthesis stack (using,for example, scroll-bar 3306), which may also be displayed on displayunit 108. In that regard, the user may use a scroll bar 3306 to scrollbetween tomosynthesis slices in the tomosynthesis stack, and thecorresponding slice number may be displayed in slice number display3304. As discussed above, the computer system 106 may generate athree-dimensional volumetric image 3906 (FIG. 39C) of the image volume.In the exemplary embodiment illustrated in FIG. 39C, thethree-dimensional volumetric image 3906 is displayed on display unit 108as opposed to a volumetric view of the image volume (e.g., FIG. 33D).However, both the three-dimensional volumetric image 3906 and avolumetric view of the image volume may be displayed on display unit 108at the same time.

As illustrated in FIG. 39C, coordinate axes 3908 may also be provided toshow the user the viewing orientation of the volumetric image. Ratherthan labeling the coordinate axes in Cartesian nomenclature, the axesmay be described using anatomical nomenclature (e.g., mesiodistal (M),coronoapical (C), and buccolingual (L)) to provide the user withinformation regarding the anatomical orientation of the image volume. Inan exemplary embodiment, the ends of the scroll bar 3306 may be labelledwith corresponding descriptions of the axes. For example, if theparticular diagnostic image results in the tomosynthesis stack beingarranged along the buccolingual axis then the ends of the scroll bar3306 may labelled “buccal” and “lingual”. Thus, the user can easilydetermine which direction to scroll the slider to move in one anatomicaldirection or another.

As described above, once the user has selected a particulartomosynthesis slice, a first marker 3910 may be placed at a firstmeasurement point (point A), as illustrated in FIG. 39B. The user maythen select a second tomosynthesis slice 4002 and place a second marker4004 to designate a second measurement point (point B), as illustratedin FIG. 40B. As described above, the second tomosynthesis slice 4002 maybe the same slice or a different slice. As discussed, the user may movethe second marker 4004 to any location on the second tomosynthesis slice4002 and the computer system 106 may dynamically calculate and updatethe magnitude of a vector 4006 from the first measurement point to thesecond measurement point. The magnitude of that vector (corresponding toa distance between the two points) may be displayed, along with themagnitudes of the individual vector components, in a text box 4008 (asillustrated in FIG. 40C). As shown in FIG. 40C, the vector 4006 may bedisplayed in partial relief, that is the vector 4006 may be partiallyshaded when the vector traverses through a structure. The vector 4006may also be shown in relief by other visual means such as, for example,a dashed line.

In an exemplary embodiment, the volumetric image 3906 may be rotated bythe user to provide different views of both the volumetric image 3906and the vector 4006 contained therein. For example, as shown in FIG.41A, the volumetric image 4006 has been rotated to provide a clearerview of the occlusal surface of the imaged tooth. The coordinate axes3908 have also been rotated to provide the user with an indication ofthe anatomical directions. In FIG. 41B, the volumetric image 3906 hasbeen rotated to provide a view of the coronoapical-mesiodistal plane(i.e., in a direction of the buccolingual axis). For simplicity, vector4006 has not been shown in relief in FIGS. 41A-41B, however, thecomputer system 106 may cause the display unit 108 to display vector4006 in relief to provide additional visualization of the vector 4006for the user.

The volumetric image 3906 may be rotated through a variety of means. Forexample if the input unit 114 is a mouse, the user may click on a partof the display volumetric image 3906 and drag the mouse to displayedvolumetric image 3906 to rotate in a corresponding manner. In anotherexample, a GUI menu may be displayed which shows one or more icons thatmay be clicked on by the user to cause the volumetric image to rotate.In yet another example, the computer system 106 may display a list ofpredetermined viewing perspectives which the user may select from. Ifthe type of diagnostic image is known, a set of corresponding viewswhich may be useful for the particular diagnostic operation may bepresented in the form a GUI menu by the computer system 106. The usermay then select one of those views which will be displayed by displayunit 108. For example, if the diagnostic operation is a measurement ofthickness of the lingual and buccal plates, a view along the mesiodistalaxis (i.e., of the buccolingual plane) may be predetermined viewselectable from the GUI menu.

Accordingly, by virtue of the example embodiments described herein, 3Dmeasurement in a stack of tomosynthesis images is provided, even acrossor within images of the stack. For example, it is possible to measurethe distances between points on objects or structures represented bytomosynthesis images, even though the images themselves are onlytwo-dimensional. Moreover, it is possible to provide useful informationbased on such measurement.

Measuring and Visualizing Lingual and Buccal Plate Thickness

As described below, methods, systems, apparatuses, and computer programproducts are provided for measuring the thicknesses of the lingual andbuccal plates.

FIG. 42A is a perspective view of the human mandible 4200. The mandible4200 includes the alveolar process 4202 (indicated as region above lineA), which is a thickened region that contains the tooth sockets. Asshown in FIG. 42B, the alveolar process 4202 includes a lingual boneplate 4204 and a buccal bone plate 4206 disposed on opposite sides of atooth cavity for tooth 4212. If a problem arises with tooth 4212 suchthat it must be replaced with an implant, the lingual plate 4204 and thebuccal plate 4206 must have sufficient strength to support the implant.If the lingual plate 4204 and the buccal plate 4206 lack sufficientstrength, a fracture may occur when the implant is exposed to forcesincurred during the chewing process. The respective strengths of thelingual plate 4204 and the buccal plate 4206 are directly related totheir respective thicknesses in a region that supports the implant.Traditional x-ray imaging in the periapical direction is incapable ofresolving the thicknesses of the lingual and buccal plates because, forone, the imaging direction (i.e., the direction in which an x-raytravels from the x-ray source) is generally perpendicular to the lingualand buccal plates. However, the imaging direction is not the only reasontraditional x-ray imaging is incapable of resolving the thicknesses ofthe lingual and buccal plates, as explained below.

FIGS. 43A and 43B are illustrations of the maxilla (FIG. 43A) andmandible (FIG. 43B) from the occlusal direction with the lingual plates4204 and 4208 and the buccal plates 4206 and 4210 shown. Unlike whenimaging in the periapical direction, here the lingual and buccal platesare substantially parallel to the imaging direction. However, asdiscussed above in reference to FIGS. 60A & 60C an traditional x-rayimage lacks depth information and overlapping objects easily obscure oneanother. For example, as shown in FIG. 60B, at least a portion of anincisor tooth 6008A and a portion of the buccal plate 4206 lie in thesame x-ray path. Thus, the incisor tooth 6012A obscures at least aportion of the buccal plate 4206, making it difficult to preciselydetermine the boundaries of the buccal plate 4206. In a similar fashiontooth 6008B obscures the view of the lingual plate 4204.

Moreover, a thickness measurement of the lingual and buccal bones fromocclusal direction using a traditional x-ray image may lead to aninaccurate measurement, as discussed below.

FIG. 60D shows cross-sectional views of the lingual plate 4204 andbuccal plate 4206, and the resulting x-ray projection images (6012 and6014) from those plates for x-rays incident in the direction of arrow C.If one were to measure the width of the white portion of images 6012 and6014 in an attempt to determine the thicknesses of the lingual plate4204 and the buccal plate 4206 (as indicated by 6016 and 6018) at aparticular depth in the imaging direction (defined by ray C), theresulting measurement may over represent the thicknesses of the plates.This because the white portions in the image represent the widest pointsof the imaged object which may not be at the same depth. This isillustrated in FIG. 60D. The thickness measurement 6016 corresponds to aplane 6020 that is angled relative to the occlusal surface of the tooth(which is generally perpendicular to the imaging direction). Thus, thelack of depth information may lead to an improper diagnosticmeasurement.

FIG. 44 is a flow chart illustrating the steps in measuring thethickness of the lingual and buccal plates, in accordance with oneexemplary embodiment.

First, in step S4402 the patient is positioned relative to the x-raysource 104. More specifically, an x-ray detector 102 (e.g., one of theintraoral sensors described above) is carefully positioned inside of apatient's mouth. The patient bites down gently on the x-ray detector 102to fix its position within the patient's mouth. A protective cover maybe placed over the x-ray detector 102 to prevent the x-ray detector 102from being damaged, as well as for sanitary reasons. Next, the x-raysource 104 is moved to an appropriate starting position for theradiographic scan (S4404). FIG. 45 is an exemplary illustration of anx-ray source 104 mounted in such a manner that it may be positioned atany location within a three-dimensional space.

As shown in FIG. 45, the x-ray source 104 may be connected to anadjustable arm 4510, which may be segmented and include one or morejoints such as: a hinge, a swivel, a universal joint, or the like. Theadjustable arm 4510 allows the x-ray source 104 to freely translate inthree-dimensional space. Attached to one end of the adjustable arm 4510is a vertical member 4520. The other end of the adjustable arm 4510 maybe mounted to a stationary structure, such as a wall or a ceiling. Thevertical member 4520 is suspended vertically from the adjustable arm4510 by a joint that allows the vertical member 4520 to freely rotateabout an axis (A1) substantially defined by the vertical member 4520,regardless of the position and orientation of the adjustable arm 4510.The vertical member 4520 includes a bearing assembly which acts as achannel through the vertical member 4520. A yoke 4530 is movablyconstrained within the channel, and can be angularly displaced throughthe bearing assembly and thus through the vertical member 4520. A brakemay hold the yoke 4530 in place and substantially prevent any motion ofthe yoke 4530 through the bearing assembly, thus locking the position ofthe yoke 4530 relative to the vertical member 4520. A brake releasebutton may also be provided such that an operator can release the brakeand allow the yoke 4530 to rotate through the vertical member 4520.

The motorized stage 118 may include arms 4560 and 4570 which are movablyattached to the yoke ends 4540 and 4550, respectively, each point ofattachment forming a pivot such that the motorized stage 118 can bepitched about an axis (A2) which is substantially defined by the yokeends 4540 and 4550 and substantially orthogonal to the axis (A3) of thex-ray source 104. In the exemplary arrangement illustrated in FIG. 45,the x-ray source 104 may be appropriately positioned at any desiredlocation in three-dimensional space such that the axis A3 of the x-raysource 104 is substantially perpendicular to the surface of the x-raydetector 102.

Returning to the discussion of step S4402, if the thicknesses of thelingual plate 4204 and the buccal plate 4206 in the mandible bone aredesired, the x-ray source 104 should preferably be positioned below thepatient's jaw, as illustrated in FIGS. 46-48. With this configuration,the teeth and the mandible (including the lingual plate 4204 and thebuccal plate 4206) comprise the object 50 and sub-objects illustrated inFIG. 1A. In an exemplary embodiment, the x-ray source 104 is initiallypositioned at the 0° position in the scan angle, which typicallycorresponds to a middle position in the scanning range (S4404). However,the x-ray source 104 may be initially positioned at any location withinthe scanning range. As shown in FIG. 47, it is preferable that a planecorresponding to the surface of the x-ray detector 102 is orthogonal tothe imaging direction of the x-ray source 104. An aiming device 4800 maybe provided to aid with the alignment of the x-ray source 104 relativeto the x-ray detector 102, as illustrated in FIG. 48.

FIG. 48 shows the x-ray source 104 which has been positioned withrespect to the x-ray detector 102 with the assistance of an aimingdevice 4800. The aiming device 4800 includes an arm 4804 which isconnected to the x-ray detector 102 and the aiming ring 4806. Theconnections between (i) the arm 4804 and the x-ray detector 102 and (ii)the arm 4804 and the aiming ring 4806 may be made by a mechanicalconnection (e.g., a snap-fit connection or a friction fit connection), amagnetic connection, an electrical connection, a chemical connection(e.g., glue), or a combination thereof. For example, the cross-sectionaldimension of the arm 4804 may be substantially the same as an opening(see opening 4904 in FIG. 49) in the aiming ring 4806 that receives thearm 4804, such that there is a significant amount of friction betweenthe opening and the arm 4804 when the arm 4804 is inserted into theopening, thus establishing a friction fit. The amount of friction issufficient to ensure that the aiming ring 4806 does not move during theradiographic scan. The arm 4804 may also extend through an opening sothat the distance between the aiming ring 4806 and the x-ray detector102 is adjustable and may be appropriately set by an operator. Once thatdistance is set, the friction between the opening and the arm 4804 willprevent the aiming ring from moving further. An opposite end of the arm4804 may include a holder that is configured to receive the x-raydetector 102 via a snap-fit connection.

The aiming ring 4806 may also include a plurality of alignment markers4902, as shown in FIG. 49. The alignment markers 4902 may be formed froma known material whose attenuation factor is known. The alignmentmarkers 4902 may be visible in the recorded two-dimensional x-rayprojection image, and can serve as reference markers during the imageprocessing. The alignment markers 4902 may also be used to measure thesystem geometry (i.e., the spatial relationship between the x-ray sourceand the x-ray detector). While a particular geometry may be assumed, theactual geometry may differ from the assumed geometry, due tomisalignment of the x-ray source, vibration or flexure of the arm of theaiming ring (or the structure on which the x-ray source is mounted), ormotion of the patient. Since, in one embodiment, the system geometry maydetermine which backprojection matrix is used and may also modify thefiltering step in a filter backprojection, the positions of thealignment markers 4902 in the recorded image can provide usefulinformation regarding the system geometry. As noted above, the aimingring 4806 may also be provided with one or more openings 4904 throughwhich the arm 4804 may be inserted to connect the aiming ring 4806 tothe x-ray detector 102.

A similar process may be performed if the thicknesses of the lingualplate 4208 and the buccal plate 4210 in the maxilla are desired. In thiscase, the x-ray source 104 may preferably be positioned above thepatient's head, as illustrated in FIGS. 50-52. Like with a scan oflingual plate 4204 and the buccal plate 4206 in the mandible, the x-raysource 104 may preferably be positioned such that the imaging directiondefined by the axis of the x-ray source 104 is substantially orthogonalto the plane of the x-ray detector 102, as illustrated in FIG. 51.

As shown in FIG. 52, an aiming device 5200 may also be used to aid inthe alignment of the x-ray source 104 with the x-ray detector 102. Likeaiming device 4800, aiming device 5200 includes an arm 5204 which isconnected to the x-ray detector 102 and the aiming ring 5206. One ormore of the components of the aiming device 5200 may be the same as thecomponents of the aiming device 4800. The components of the aimingdevice 5200, however, may also be different from those included inaiming device 4800, and specifically constructed for use in imaging thelingual plate 4208 and the buccal plate 4210 of the maxilla from anocclusal direction. Like with aiming device 4800, the connectionsbetween (i) the arm 5204 and the x-ray detector 102 and (ii) the arm5204 and the aiming ring 5206 may be made by a mechanical connection(e.g., a snap-fit connection or a friction fit connection), a magneticconnection, an electrical connection, a chemical connection (e.g.,glue), or a combination thereof. Like with aiming device 4800, thedistance between the aiming ring 5206 and the x-ray detector 102 may beadjustable.

With the x-ray source 104 properly aligned with the x-ray detector 102,the computer system 106 initiates the scanning operation to record aplurality of projection images in step S4406. As discussed above,on-board motor controller 120 controls the motorized stage 118 so as totranslate the x-ray source 104 through a plurality of locations withinthe scan angle 112, as illustrated in FIG. 46.

FIG. 46 illustrates the translation (and rotation) of the x-ray source104 during an imaging operation over a scan angle of X degrees (whichmay, for example, be ±20°). In FIG. 46, the x-ray source 104 translatesalong the motorized stage 118, but also rotates such that the x-rays 110emitted from each of the different locations converge substantially at afocal spot 122, as illustrated in FIG. 1A. Of course, in anotherembodiment, the x-ray source 104 may be moved in a curved scan path 132or a circular scan path 134, as described above.

At each location, the x-ray detector 102 records the intensity of thex-rays 110 received from the x-ray source 104 and converts those x-rays110 into electrical signals. The computer system 106 then processesthose electrical signals to generate a two-dimensional projection image.The x-ray source 104 then moves to the next position and a subsequenttwo-dimensional projection image is recorded. This process repeats ateach desired scan location over the scanning range to generate aplurality of two-dimensional projection images.

In step S4408, the computer system 106 processes the plurality ofprojection images to reconstruct a series of two-dimensionaltomosynthesis image slices that extend in a depth direction (the Z-axisin FIGS. 46-38 and 50-52), in the manner described above.

In step S4410, a particular slice corresponding to a desired depth inthe depth direction may be selected. As discussed above, an operator maywish to know the thickness of the lingual and buccal plates at aparticular depth or depths. For example, as illustrated in FIGS. 53A and53B, the operator may wish to know the thicknesses at depths D₁ and D₂,which are within the depth of the tooth cavity. The operator may inputone or more desired depths via the input unit 114 (or the display unit108 if a touchscreen), and the computer system 106 may return atwo-dimensional image corresponding to the desired depth, as illustratedin FIG. 53C for a depth D₁. Unlike a traditional two-dimensional x-rayimage, the two-dimensional image shown in FIG. 53C is substantiallyparallel to the plane of the x-ray detector 102 at the depth D₁, andthus generally perpendicular to the lingual and buccal plates. As such,the thicknesses of the lingual and buccal plates can be determined bydirect measurement using the two-dimensional image (S4412).

More particularly, in step S4412, an operator may select two points inthe two-dimensional image and the computer system 106 will automaticallyreturn the distance between those points in a desired unit. In oneembodiment, the operator may draw a line between two points (asillustrated in FIG. 53C) and the computer system 106 will return thedistance between the end points of the line. For example, assume anoperator enters a depth D₁, corresponding to a depth which is roughlyone-third of the depth of the tooth cavity shown in FIGS. 52A and 52B(Note: the trabecular region is omitted from FIGS. 52A and 52B forsimplicity purposes). The computer system 106 will return and displaythe two-dimensional image shown in FIG. 53C on a display unit 108. InFIG. 53C, the lingual plate 4204 and the buccal plate 4206 are clearlyvisible. As shown in FIG. 53C, an operator may draw lines 5302 and 5304across the lingual plate 4204 and the buccal plate 4206, respectively,and the computer system 106 will calculate and display the length ofthose lines (5302 and 5304) in order to determine the thicknesses (T_(L)and T_(B)) of the lingual plate 4204 (T_(L)) and the buccal plate 4206(T_(B)) at that depth. By redrawing lines 5302 and 5304 at anotherlocation, the operator may determine the thicknesses of the lingualplate 4204 and the buccal plate 4206 at that location as well. If theoperator wishes to make another thickness measurement at a second depth(e.g., depth D2 shown in FIGS. 53A and 53B), the operator need onlyprovide the tomosynthesis system 100 with the desired depth informationvia the input unit 114, and the computer system 106 will return acorresponding two-dimensional image at that depth from which theoperator can select points or draw a line so as to measure thethicknesses of the lingual plate 4204 (T_(L)′) and the buccal plate 4206(T_(B)′). Thus, contrary to a traditional x-ray imaging, the abovemethod allows the operator to determine the thicknesses of the lingualand buccal plates at a desired depth, without subjecting the patient toa relatively large dose of radiation, which they may receive from a CBCTscan.

As noted above, the tomosynthesis system 100 may receive guidance fromthe operator indicating a clinical aspect of interest, which may be thelingual and buccal plates. To aid the operator in identifying thelingual and buccal plates at a particular depth, the computer system 106may analyze the tomosynthesis stack and automatically segment theanatomical features therein. In other words, the computer system 106 mayautomatically determine which portion of one or more two-dimensionalimages correspond to the clinical aspect of interest (i.e., the lingualand buccal plates). In one embodiment, the computer system 106 may userelative attenuation factors, determined from analyzing the plurality ofprojection images, to discern the presence of one or more objects withinthe tomosynthesis stack and segment those objects from each other. Thecomputer system 106 may then highlight, outline, or color code thosesegmented objects and display them on the display unit 108. In oneembodiment, the computer system 106, having segmented the lingual andbuccal plates from the surrounding objects, may automatically measurethe thicknesses of those plates at one or more locations on thetwo-dimensional image and display those measurements on the display unit108. In an exemplary embodiment, a user may provide the computer system106 with information regarding the location of a desired implant and adepth range for the implant. In return the computer system 106 mayprovide measurements of the plate thicknesses at one or more depthswithin that range. Of course, this does not preclude the operator fromselecting a different set of points, or drawing a different line, at adesired location to measure the thickness of the lingual or buccal plateat that location.

Tracking Motion from Projection Images

The intraoral tomosynthesis system 100 will now be further described inconjunction with FIG. 54, which is a flow diagram illustrating a processof tracking motion of one or more objects using a plurality ofprojection images in an intraoral dataset according to an exampleembodiment herein. In one example embodiment, the process steps shown inFIG. 54 can be embodied in a control program stored in a non-transitorycomputer-readable storage medium and loaded into the main memory 232and/or the secondary memory 234 of the computer system 106 using theremovable-storage drive 238, the hard disk drive 236, and/or thecommunications interface 246. Control logic (software), when executed bythe processor 222, causes the computer system 106, and more generallythe intraoral tomosynthesis system 100, to perform the proceduresdescribed herein.

Prior to starting the process illustrated in FIG. 54, the x-ray detector102 and x-ray source 104 are first aligned. Typically, the x-raydetector 102 and the x-ray source 104 are manually aligned by aclinician. In that regard, an aiming device may be used to ensure aproper initial alignment. However, relative motion of the x-ray detector102 and the x-ray source 104 may occur during a scanning operation as aresult of one or more factors including, for example, movement of object50 (e.g., a patient), sub-object 52 (e.g., an object inside thepatient), x-ray source 104, and/or x-ray detector 102. FIG. 54 describesan exemplary method for tracking such relative motion.

In Step S5402, the intraoral tomosynthesis system 100 acquires aplurality of projection images of object 50 over a scan angle range 112(which may be predetermined), including the orthogonal projection image,in the manner described above. For example, the x-ray source 104 ismoved by the motorized stage 118 and control circuitry 120 to differentpositions within the scan angle 112, and the computer system 106controls the x-ray source 104 to emit x-rays 110 at each position. Asdiscussed above, and shown in FIG. 1A, the x-ray source 104 maytranslate as well as rotate about a pivot point. In one exampleembodiment herein, the scan angle 112 ranges from −20° to +20°, with thescanning positions evenly distributed in increments of 0.8° to provide51 scan angles, including the 0° position, although this example is notlimiting. The x-rays 110 then pass through and are attenuated by theobject 50 before being projected onto the x-ray detector 102. The x-raydetector 102 converts the x-rays 110 into electrical signals (eitherdirectly or indirectly, as described above) and provides the electricalsignals to the computer system 106. The computer system 106 processesthe electrical signals collected at each scan angle position to acquirethe plurality of projection images, each image comprising an array ofpixels. The image acquired with the x-ray source 104 at the 0° positionis also referred to herein as an orthogonal projection image.

In one example embodiment herein, the color depth of each pixel value ofthe projection images may be 12-bit grayscale, and the dimensions of theprojection images correspond to the standard dental size of the x-raydetector 102, as described above. For example, a Size-2 detector mayproduce projection images that are approximately 1700×2400 pixels insize, a Size-1 detector may produce projection images that areapproximately 1300×2000 pixels in size, and a Size-0 detector mayproduce projection images that are approximately 1200×1600 pixels insize.

In Step S5402, the computer system 106 generates a sub-sample ofprojection images from the plurality of projection images obtained instep S5402. The sub-sample of projection images may be stored in, forexample, main memory 232 or secondary memory 234. The sub-sample ofprojection images is for use in a coarse image reconstruction, performedin step S5406. By using a sub-sample of projection images, lesscomputational resources and time are consumed by the imagereconstruction process in step S5406. Of course, in an alternateembodiment, step S5404 may be skipped and the reconstruction process instep S5406 may be conducted using all of the projection images obtainedin step S5404.

The sub-sample of projection images may include less than all of theprojection images obtained in step S5402 (e.g., 25 out of 51 projectionimages may be included in the sub-sample of projection images). In oneembodiment, for example, every other projection image in the pluralityof projection images is included in the sub-sample of projection images.Preferably, at least half of the projection images are included in thesub-sample of projection images. Each individual projection image in thesub-sample of the projection images may be further sampled to reduce thenumber of pixels therein. For example, in one embodiment, half of thepixels in each projection image are filtered out (e.g., every otherpixel in the projection image). This further reduces the computationalintensity of the reconstruction process in step S5406. Of course, inanother embodiment, all of the plurality of projection images may beretained for use in the reconstruction process in step S5406, but thenumber of pixels in each projection image may be reduced.

In step S5406, image reconstruction is performed using the sub-sample ofprojection images obtained in step S5404. This image reconstruction isconsidered a coarse reconstruction because less than all of theprojection images and/or pixels are used in the reconstruction process.Computer system 106 may also perform deblurring and other imageenhancements, as will be described further herein.

As discussed above, each reconstructed tomosynthesis image slice iscomprised of an array of pixels that represent a cross-section of object50 in a plane that is parallel to the surface of the x-ray detector 102(the x-y plane in FIG. 1A). The plane is located a certain distance fromthe x-ray detector 102 in a direction that is orthogonal to the surfaceof the x-ray detector 102 (the z-axis direction in FIG. 1A).

Each tomosynthesis image slice has a certain thickness along the z-axisthat is a function of the reconstruction technique used to create thetomosynthesis image slices and aspects of the geometry of the system100, including, primarily, the scan angle 112. For example, eachtomosynthesis image slice may have a slice thickness of 0.5 mm by virtueof the geometry of the system 100 and the reconstruction technique. Thedesired location of each tomosynthesis image slice along the z-axis isprovided as an input to the reconstruction performed in Step S5406either as a pre-programmed parameter in computer system 106 or by userinput via input unit 114 and/or display unit 108. By example only, thecomputer system 106 can be instructed to reconstruct, from the pluralityof projection images, a first tomosynthesis image slice that is onemillimeter (1 mm) away from the surface of x-ray detector 102 along thez-axis, a last tomosynthesis image slice that is fifteen millimeters (15mm) away from the surface of the x-ray detector 102, and one or moreimage slices between the first image slice and the last image slice atregular increments along the z-axis of two-hundred micrometers (200 μm),for a total of seventy-one image slices.

Reconstruction of the tomosynthesis image slices in Step S5406 may beperformed in accordance with any existing or later developedreconstruction techniques. One exemplary technique which may be used isthe shift-and-add technique. The shift-and-add technique utilizesinformation about the depth of sub-object(s) 52 along the z-axis that isreflected in the parallax captured by the plurality of projectionimages, as described above. According to this example embodiment, animage slice is reconstructed by first spatially shifting each projectionimage by an amount that is geometrically related to the distance betweenthe image slice and the tomographic focal spot 122 along the z-axis. Theshifted projection images are then averaged together to result in theimage slice, where all sub-objects 52 in the plane of the image sliceare in focus and sub-objects 52 outside of that plane are out of focusand blurry. This shift-and-add process is repeated for each image sliceto be reconstructed. In the case of the image slice corresponding to thex-y plane that includes the focal spot 122, the projection images areaveraged together without first shifting because sub-objects 52 arealready in focus for that plane.

As mentioned above, a deblurring technique may be used to deblurreconstructed tomosynthesis image slices. In one example embodimentherein, a deblurring technique that substantially reduces or removesblurry, out-of-plane sub-objects from an image slice can be performed inconjunction with the reconstruction technique (whether shift-and-add oranother technique). Examples of deblurring techniques that can beemployed are, for example, spatial frequency filtering, ectomography,filtered backprojection, selective plane removal, iterative restoration,and matrix inversion tomosynthesis, each of which may be used in StepS5404 to deblur images reconstructed by the shift-and-add reconstructiontechnique (or another reconstruction technique, if employed).

In another example embodiment herein, Step S5406 also can include thecomputer system 106 performing further automated image enhancements suchas, for example, image sharpening, brightness optimization, and/orcontrast optimization, on each reconstructed (and deblurred, wheredeblurring is performed) image slice in a known manner.

Additionally, in another example embodiment herein, the dimensions,position, and orientation of each image slice reconstructed in StepS5406 are the same as the corresponding characteristics of theorthogonal projection image. Thus, when tomosynthesis image slices (orportions thereof) and the orthogonal projection image are overlaid overone another, corresponding anatomical features appearing in the imageswill be overlapped and aligned without scaling, rotation, or othertransformation of the images.

In Step S5408, the computer system 106 processes the plurality ofprojection images acquired in step S5402 to identify one or more objectsin each projection image that may be appropriate for tracking motion.This process is detailed in FIG. 55.

As noted above, each projection image is comprised of a plurality ofpixels, each with an assigned color depth (e.g., 12-bit grayscale). Thetotal number of pixels in each projection image is dependent upon thesize of the x-ray detector 102, as discussed above. In a preferredembodiment, each of the projection images acquired in step S5402 has thesame number of pixels contained therein. In step S5502, a differencebetween neighboring projection images is calculated on a pixel-per-pixelbasis, as shown in FIGS. 56A-57B.

FIG. 56A shows a first projection image 5600 recorded at a firstscanning position in the scan angle 112. FIG. 56B is a second projectionimage 5602 recorded at a second scanning position in the scan angle 112.The first and second scanning positions are adjacent to one another inthe scan angle 112. FIG. 57A is a difference image 5700, representingthe difference between pixel values at the same x and y locations in thefirst and second projection images. The pixel values in the differenceimage (representing the difference between the first and secondprojection images) may be label as a set of values D₂(x,y). While FIG.57A shows a difference image 5700 corresponding to the first and secondprojection images, computer system 106 calculates a difference image foreach adjacent pair of projection images (that is projection imagescorresponding to adjacent scanning positions) to create a seriesdifference images with pixel values D_(ij)(x,y).

The difference images calculated in S5502 may be used to identifyobjects for motion tracking. Alternative methods may be used to identifythe objects for motion tracking. For example, a thresholding imagemethod (S5504-S5506) or a gradient segmentation method (S5508-S5514) maybe used. Briefly, in the thresholding image method (S5504-S5506),described in further detail below, a difference image 5700 is used togenerate a binary image 5700 (see FIG. 57B), which is subsequentlyanalyzed to determine connected segments. One or more of the connectedsegments (e.g., the four largest connected segments) are identified asobjects for motion tracking. The computer system 106 then generates aregion of interest (ROI) around the identified objects (S5516).

In the gradient segmentation method (S5508-S5514), described in detailbelow, a gradient segmentation operation is performed on the differenceimage 5700 (S5508). The gradient segmentation operation calculates themagnitude and direction of the gradient at each pixel location in thedifference image 5700. The computer system 106 isolates the vertical andhorizontal edges (S5510), based on the calculated gradient, andidentifies the vertical and horizontal edges with a gradient magnitudeabove a certain magnitude (S5512). Of those vertical and horizontaledges, certain edges are selected as objects for motion tracking(S5514). The computer system 106 then generates a region of interest(ROI) around the identified objects (S5516).

Turning to the thresholding image method, in step S5504 a thresholdingoperation is performed on the pixel values in the difference imageD_(ij)(x,y) to produce a binary image. More specifically, each pixelvalue in D_(ij)(x,y) is compared to a threshold value. If the pixelvalue is greater than threshold value, then a pixel value at acorresponding point in a binary image (represented by a series of valuesB_(ij)(x,y)) is set to 1, otherwise the pixel value at the correspondingpoint is set to 0. In one embodiment, the threshold value may be set asthe average pixel value in the difference image, resulting in half ofthe pixels in the binary image having a value of 1, and the other halfhaving a value of zero. An exemplary binary image 5702 is shown in FIG.57B.

Once the binary image 5702 is generated in step S5504, the binary image5702, or more specifically the values B_(ij)(x,y) comprising the binary5702 image, are analyzed to determine connected segments (step S5506).This may be accomplished by analyzing the binary image 5702 to determineregions where values of “1” are continuously adjacent to one another.The largest connected segments in the binary image (i.e., the longestcontinuous chain of adjacent values of “1”) are then identified bycomputer system 106, and one or more of those segments are identified asobjects for motion tracking. In one embodiment, the computer system 106identifies the four largest objects as objects for motion tracking;however, more or less objects may be used. As shown in FIG. 57B, thecomputer system 106 has identified four objects 5704-5707 for motiontracking from the binary image 5702.

Turning to the gradient segmentation method, in step S5508 the gradientof the difference image 5700 is calculated from the values D_(ij)(x,y).The computer system 106 analyzes the direction of the gradient on apixel level basis to identify vertical and horizontal edges. Forexample, the computer system 106 analyzes adjacent pixels to determineif the direction of the gradient is substantially vertical or horizontal(e.g., within a range of 10 degrees of either vertical or horizontal).If a pixel falls outside of this range, it is disregarded. If thedirection of the gradient is substantially vertical for adjacent pixels,then the adjacent pixels may correspond to a horizontal edge. If thedirection of the gradient is substantially horizontal then the adjacentpixels may correspond to a vertical edge. The computer system 106 maydetermine the presence of a vertical or horizontal edge if the gradientfor a minimum number of adjacent pixels is in the horizontal or verticaldirections, respectively. In step S5512, the computer system 106analyzes the identified vertical and horizontal edges to determine themagnitude of the gradient along those edges. In step S5514, the computersystem 106 identifies vertical and horizontal edges where the magnitudeof the gradient is above a certain threshold. For those vertical andhorizontal edges where the magnitude of the gradient is above thethreshold, the computer system 106 determines the size of the edges andselects one or more of the largest edges as objects for motion tracking.In one embodiment, edges comprised of 200 or more pixels are used formotion tracking.

Regardless of whether the objects for motion tracking are determined bythe thresholding image method or the gradient segmentation method, oncethe objects for motion tracking are identified, the computer system 106automatically generates regions of interest (ROIs) around the objects,in step S5516. For example, FIG. 58 is a projection image where fourregions of interest 5801-5804 have been generated around the positionsof objects 5704-5707 in FIG. 57B.

After Step S5408 is performed, control passes to step S5410 in which thecomputer system 106 applies a tracking algorithm for each region ofinterest. The tracking algorithm is explained in further detail below,with reference to FIG. 59. In one embodiment, one or more processes(e.g., a filtering operation) may be performed prior to executing thetracking algorithm. For example, the pixel values in projection imagesmay be filtered.

FIG. 59 shows two projection images (5902 and 5904) recorded at adjacentscanning positions within the scan angle 112. As shown in FIG. 59, theposition of object 5906 is shifted up and to the left in the secondprojection image 5904. To determine the amount of shift, a region ofinterest 5908 may be generated over a portion of object 5906 in thefirst projection image 5902. In one embodiment, the region of interest5908 may be windowed to reduce edge effects. The computer system 106then overlays the region of interest 5908 at a first position (P₁) inthe second projection image 5904, and multiplies the pixels value fromthe region of interest 5908 in the first projection image 5902(P_(ROI)(x,y)) by the pixels values corresponding to the area of thesecond projection image which the region of interest 5908 overlays(P_(5902-P1)(x,y)), respectively, and then sums the result. The sum is across-correlation function. The summation (i.e., the cross-correlationfunction) is recorded and stored in memory 232 or 234. The region ofinterest 5908 is then translated to a second position and anothermultiplication is calculated and the corresponding summation isrecorded. In one embodiment, the region of interest 5908 is translatedby a unit of one pixel in either the x or y direction. However, theregion of interest 5908 may be translated by a greater amount to reducecomputational time. The region of interest is thus translated across thesecond projection image 5904, and a series of summations (of theplurality of individual multiplications) are recorded. When the regionof interest 5908 is overlaid with the same portion of the object 5906that the region of interest 5908 corresponds to in the first projectionimage 5902 (at an alignment position P_(A)), the resultingmultiplication produce a relatively large result, and thus the summationof those multiplications is also relatively large and identifiable bythe computer system 106. In other words, the computer system 106determines the translation for which the cross-correlation function is amaximum as the alignment position. For example, assume the projectionimage is comprised of nine pixels arranged in a 3×3 array, with thecenter pixel having a value of pure white (e.g., 255) and the remainingpixels have a value of pure black (i.e., 0). If the ROI is the centerpixel, then the pixel value for the ROI is also pure white (e.g., 255).Note, since the ROI only contains one pixel, there is only onemultiplication operation and no summation operation in this example. Theresult of a multiplication of the pixel value corresponding to the ROI(255) by any pixel other than the center pixel (0) is 0. When the pixelvalue corresponding to the ROI (255) is multiplied by the center pixel(255), however, the result is 65,025. This result is a maximum among thecalculations and thus corresponds to an alignment position. Since thetranslations of the ROI are approximately known, the possible alignmentposition is restricted to a set of possible alignment positions. Inanother embodiment, the cross-correlation function may be determined byapplying a fourier transform technique.

With the alignment position and the position of the region of interestin the first projection image known, the computer system 106 maydetermine how much shift occurred in both the x and y directions betweenthe first projection image 5902 and the second projection image 5904.

As noted above, the computer system 106 may track the motion of morethan one object. As such, the above tracking algorithm may be performedfor each ROI corresponding to the objects for motion tracking (e.g.,objects 5704-5707 in FIG. 57B). If the motion of more than one object istracked, then (in step S5412) an average pixel shift may be determinedfrom the calculated shifts for each ROI, with the average pixel shiftalong the x and y axes calculated separately.

Through the above-described process, an average shift for each ROIbetween two projections images may be determined. This process may beused to calculate the average pixel shift for ROIs between projectionimages obtained at adjacent scanning positions across the scan angle112. For example, an average ROI shift (ROI₁₂) between a firstprojection image (corresponding to scan angle of 20°) and a secondprojection image (corresponding to scan angle of 19.2°) may becalculated, as well as an average ROI shift (ROI₂₃) between the secondprojection image (corresponding to the scan angle of 19.2°) and a thirdprojection image (corresponding to a scan angle of 18.6°).

Once all of the ROI shifts are calculated across the scan angle 112, itis necessary to shift each ROI in each projection image by thecumulative shift amount in order for all of the ROIs to align in thetomosynthesis stack.

The cumulative shift is the sum of the ROI shifts that occur in the xand y direction between the orthogonal imaging position (correspondingto 0° in the scan angle, e.g. SP_(0x)) and a particular scanningposition. For example, assume there are 51 scanning positions within thescan angle 112. The cumulative ROI (along the x-axis) shift for scanningposition SP⁻² (corresponding one end of the scan angle 112), is the sumof the cumulative ROI shifts between scanning positions SP_(−25x),SP_(−24x), SP_(−23x), . . . , to SP_(−1x). In a similar manner, thecumulative ROI shift for scanning position SP⁻¹³, along the x axis isthe sum of the cumulative shifts between scanning positions SP⁻¹³,SP_(−12x), . . . , to SP_(−1x). In step S5414, the computer system 106shifts each ROI in each projection image by the appropriate cumulativeshift amount.

After Step S5414, control passes to Step S5416 which takes as input thecoarse tomographic image slices reconstructed in step S5404 and theshifted ROIs of step S5414. In step S5416, the computer system 106determines the plane of focus for each object used for motion trackingusing, for example, the focus technique described above. In oneembodiment, computer system 106 evaluates how in focus the ROI is foreach slice in the tomosynthesis stack and assigns a corresponding focusfactor. The computer system 106 determines the tomosynthesis slice withat least a local maximum focus factor which corresponds to the plane inwhich the object is in focus. By determining the plane in which theobject is in focus, the location of the object relative to the surfaceof x-ray detector 102 and the x-ray source 104 can be determined. Inother words, the distance in the imaging direction (the z-axis) fromtomosynthesis image slice that contains a high-focus image of the objectto the x-ray source 104 is used as the distance in the imaging directionfrom the object(s) for motion tracking to the x-ray source 104. Withthis information, the actual shift amount for the x-ray source betweentwo projection images corresponding to adjacent scanning positions maybe determined (step S5418).

It should be noted that the same object may not be used to determine theactual shift amount for each pair of projection images. An object usedfor motion tracking between a first and second projection image, may notbe the same object used for motion tracking between a second and thirdprojection image. If a different object is used, then the distance fromthe x-ray source 104 to the different object in the imaging directionwill likely change. Nevertheless, the above process takes this fact intoconsideration, and thus produces actual shift amounts for the x-raysource between the projection images regardless of whether the sameobject is used for motion tracking or not.

In step S5420, the actual shift amounts for the x-ray source may becompared to theoretical shift amounts. As mentioned above, motion of thepatient, the x-ray detector 102, and/or the x-ray source 104, may causethe actual shift amounts to depart from the theoretical shift amount,which assumes that the system geometry (the spatial relationship betweenthe patient, x-ray detector 102, and x-ray source 104) is constant.However, this comparison is not necessary to generate motion-compensatedtomographic images, and is therefore an optional step.

In step S5422, the scan angles (which are one input for thereconstruction algorithm) are updated based on the actual shift amountsfor the x-ray source 104 for motion tracking. Corrections to the scanangles may be calculated based on the actual shift amounts and the pixelsize (in mm) for a given x-ray detector 102. The updated scan angles arethen input into the reconstruction algorithm (step S5424). The computersystem 106 then performs image reconstruction on the motion-compensatedprojection images (step S5426).

The motion-compensated tomographic images may then be displayed ondisplay unit 108. In one example embodiment herein, the displaying canbe performed as to show the entire stack, or one or more selected imageslices of the stack, using display unit 108, and interactive controls(e.g. via display unit 108 and/or input device 114) enable a user toselect between those two options, and to select one or more image slicesfor display, and also to select one or more particular regions ofinterest in the image(s) for display (whether in zoom or non-zoom, orreduced fashion). In a further example embodiment, stack controls areprovided and can include a scroll bar, which enables the user tomanually select which image slice is displayed on the display unit 108,and/or can include selectable control items, such as play, pause, skipforward, and skip backward, (not shown) to enable the user to controlautomatic display of the tomosynthesis stack, as a cine loop forexample, on the display unit 108.

As will be appreciated by those of skill in the relevant art(s) in viewof this description, the example aspects described herein can beimplemented using a single computer or using a computer system thatincludes multiple computers each programmed with control logic toperform various of the above-described functions.

The various embodiments described above have been presented by way ofexample and not limitation. It will be apparent to persons skilled inthe relevant art(s) that various changes in form and detail can be madetherein (e.g., different hardware, communications protocols, and thelike) without departing from the spirit and scope of the presentinvention. Thus, the present invention should not be limited by any ofthe above-described exemplary embodiments, but should be defined only inaccordance with the following claims and their equivalents.

In addition, it should be understood that the attached drawings, whichhighlight functionality described herein, are presented as illustrativeexamples. The architecture of the present invention is sufficientlyflexible and configurable, such that it can be utilized and navigated inways other than that shown in the drawings.

Moreover, the example embodiments described herein are not limited tointraoral tomosynthesis imaging. The example embodiments describedherein can be used to perform scans of other anatomical regions. Inaddition, one or more of the above techniques may also be applied totomography in general (including CT and CBCT).

Further, the purpose of the Abstract is to enable the U.S. Patent andTrademark Office and the public generally, and especially scientists,engineers, and practitioners in the relevant art(s), who are notfamiliar with patent or legal terms and/or phraseology, to determinequickly from a cursory inspection the nature and essence of thetechnical subject matter disclosed herein. The Abstract is not intendedto be limiting as to the scope of the present invention in any way. Itis also to be understood that the procedures recited in the claims neednot be performed in the order presented.

What is claimed is:
 1. A method for reducing tomographic imagereconstruction artifacts, the method comprising: acquiring a pluralityof projection images; reconstructing a plurality of tomographic imagesfrom the plurality of projection images; generating a mask from at leastone of the plurality of projection images; isolating regions of theplurality of tomographic images using the mask; and compressing pixelvalues in the regions of the plurality of tomographic images isolated inthe isolating to generate reduced-artifact tomographic images.
 2. Themethod according to claim 1, wherein the acquiring includes performingtomosynthesis x-ray imaging, and the tomographic images aretomosynthesis images.
 3. The method according to claim 1, wherein the atleast one of the plurality of projection images is an orthogonalprojection image, and the generating includes forming the mask fromareas of the orthogonal projection image that have a low pixel varianceand a high pixel value.
 4. The method according to claim 1, wherein thecompressing includes applying a low contrast intensity transformationfunction to reduce a visibility of artifacts in the plurality oftomographic images.
 5. The method according to claim 1, furthercomprising applying a contrast enhancing intensity transformation topixel values in portions of the plurality of tomographic images notisolated in the isolating.
 6. The method according to claim 5, whereinthe contrast-enhancing intensity transformation is a linear intensitytransformation, a sinusoidal intensity transformation, or a logarithmicintensity transformation.
 7. The method according to claim 1, furthercomprising processing the plurality of tomographic images by performingat least one of an image blurring, an image sharpening, a brightnessoptimization, and a contrast optimization to improve an image quality ofthe plurality of tomographic images.
 8. The method according to claim 1,further comprising displaying at least one of the reduced-artifacttomographic images on a display unit.
 9. The method according to claim2, wherein the mask corresponds to areas of the orthogonal projectionimage deemed to not include anatomic or diagnostic information.
 10. Asystem for reducing tomosynthesis image reconstruction artifacts, thesystem comprising: a memory, storing a control program; and acontroller, operating under control of the control program to: acquire aplurality of projection images, reconstruct a plurality of tomographicimages from the plurality of projection images, generate a mask from atleast one of the plurality of projection images, isolate regions of theplurality of tomographic images using the mask, and compress pixelvalues in the regions of the plurality of tomographic images isolatedusing the mask to generate reduced-artifact tomographic images.
 11. Thesystem according to claim 10, wherein the plurality of projection imagesare x-ray images, and the tomographic images are tomosynthesis images.12. The system according to claim 10, wherein the at least one of theplurality of projection images is an orthogonal projection image, andthe controller also operates under control of the control program toform the mask from areas of the orthogonal projection image that have alow pixel variance and a high pixel value.
 13. The system according toclaim 10, wherein the controller compresses pixel values by applying alow contrast intensity transformation function to the pixel values toreduce a visibility of artifacts in the plurality of tomographic images.14. The system according to claim 10, wherein the controller alsooperates under control of the control program to apply acontrast-enhancing intensity transformation to pixel values innon-isolated portions of the plurality of tomographic images.
 15. Themethod according to claim 14, wherein the contrast-enhancing intensitytransformation is a linear intensity transformation, logarithmicintensity transformation or a sinusoidal intensity transformation. 16.The system according to claim 10, wherein the controller also operatesunder control of the control program to process the plurality oftomographic images by performing at least one of an image blurring, animage sharpening, a brightness optimization, and a contrast optimizationto improve an image quality of the plurality of tomographic images. 17.The system according to claim 10, wherein the controller also operatesunder control of the control program to provide the at least onereduced-artifact tomographic image to a display unit.
 18. The systemaccording to claim 12, wherein the mask corresponds to areas of theorthogonal projection image deemed to not include anatomic or diagnosticinformation.
 19. A non-transitory computer-readable storage mediumstoring a program which, when executed by a computer system, causes thecomputer system to perform a procedure comprising: acquiring a pluralityof projection images; reconstructing a plurality of tomographic imagesfrom the plurality of projection images; generating a mask from at leastone of the plurality of projection images; isolating regions of theplurality of tomographic images using the mask; and compressing pixelvalues in the regions of the plurality of tomographic images isolated inthe isolating to generate reduced-artifact tomographic images.
 20. Thenon-transitory computer-readable storage medium according to claim 19,wherein the at least one of the plurality of projection images is anorthogonal projection image, and the generating includes forming themask from areas of the orthogonal projection image that have a low pixelvariance and a high pixel value.